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Title Study on Atomic and Local Electronic Structures of Fe3O4(001) Film Surfaces : Clean and Modified by Adsorbed HAtom

Author(s) 樋浦, 諭志

Issue Date 2016-12-26

DOI 10.14943/doctoral.k12483

Doc URL http://hdl.handle.net/2115/64446

Type theses (doctoral)

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Study on Atomic and Local Electronic

Structures of Fe3O4(001) Film Surfaces:

Clean and Modified by Adsorbed H Atom

A ThesisSubmitted to The Graduate School of Information Science and Technology

and The Committee on Graduate Studies of Hokkaido UniversityIn Partial Fulfillment of the Requirementsfor the Degree of Doctor of Philosophy

By

Satoshi Hiura

December 2016

Nanoelectronics Laboratory

Graduate School of Information Science and Technology

Hokkaido University

Sapporo, JAPAN

I certify that I have read this dissertation and that in my opinion

it is fully adequate, in scope and quality, as a dissertation for the

degree of Doctor of Philosophy.

Prof. K. Sueoka (Principal adviser)

Prof. Y. Takahashi

Prof. T. Uemura

Prof. A. Murayama

Approved by the University Committee on Graduate Studies

i

Acknowledgements

　

I would like to thank Professor Kazuhisa Sueoka, my supervisor, for providing

me with the opportunity to perform surface science research of magnetic iron oxide

at a high level of excellence. In any respect he provided kind supports for doing

experiments, technical solutions, planning of research, data analysis, writing papers

and so on. He always treated me gently and gave me valuable advises.

I would like to thank Professor Yasuo Takahashi, Professor Tetsuya Uemura and

Professor Akihiro Murayama for serving as a reviewer of this doctoral thesis.

I also would like to thank Professor Emeritus Masafumi Yamamoto for stimulating

discussions and helpful suggestions in particular when making out applications for

JSPS Research Fellowships for Young Scientists. Thanks to him, I could greatly

mature.

I am also grateful to Associate Professor Takaaki Koga, Assistant Professor Eiji

Hatta and Dr. Hirotaka Hosoi for all valuable advises and discussions.

I also sincerely wish to thank Dr. Agus Subagyo for teaching me fundamental parts

of surface science experiments, technical solutions and how to plan research work.

Whenever I asked him something, he friendly gave me valuable advises. He has a

detailed knowledge of various fields, such as nanoscale measurement, nanofabrication

and fundamental physics. So, I think he is a “superman” of our laboratory.

I also would like to thank deeply Dr. Akira Ikeuchi for his support when I was

doing experiments in the Nanoelectronics laboratory. Whenever I asked some ques-

tions, he gave me shrewd advises with a smile. And, he organized a variety of

ii

Acknowledgements 　

laboratory events such as going bowling, karaoke, marathons, drinking parties and

so on. I thought he was the best student leader of the Nanoelectronics laboratory.

I would like to thank Mr. Masafumi Jochi for giving me all valuable discussions

and supporting me for operating and maintaining ultra-high vacuum surface sci-

ence system. In addition, we enjoyed drinking alcohols and casual conversations in

laboratory events.

I also would like to thank Mr. Atsushi Sawada for giving me all valuable advises,

discussions and enjoyable laboratory life. We entered into the Nanoelectronics labo-

ratory in the same period and have worked hard together to raise its research level.

He is talented about theoretical physics and simulation techniques, so I am looking

forward to his playing active role in a private company in next year.

I would like to be grateful to Ms. Michiyo Kanoh and Ms. Yukari Tatsumi for

their constant care about me and support concerning office procedures.

I would like to thank all members of Nanoelectronics laboratory for giving me a

comfortable environment for advancing my research. Casual conversations, drinking

parties, going bowling and marathons with them made my laboratory life enjoyable

through my PhD course.

Finally, I would like to sincerely thank my parents, Masaru Hiura and Chiemi

Hiura. I fell sure that I couldn’t finish my PhD course without their support and

generous encouragement.

iii

Table of Contents

　

Acknowledgements　 ii

Table of Contents　 iv

List of Figures 　 vii

Chapter 1 Introduction 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Spintronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Half-metallic material . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.3 Characteristics of magnetite (Fe3O4) . . . . . . . . . . . . . . 4

1.1.4 Disappearance of half-metallicity at the Fe3O4(001) surface . . 5

1.1.5 Recovery of surface half-metallicity by hydrogen adsorption . . 6

1.2 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Chapter 2 Experimental setup 9

2.1 UHV surface science system . . . . . . . . . . . . . . . . . . . . . . . 9

2.2 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.1 Scanning tunneling microscopy (STM) . . . . . . . . . . . . . 11

2.2.2 Scanning tunneling spectroscopy (STS) . . . . . . . . . . . . . 16

2.2.3 Differential tunneling conductance (dI/dV) mapping . . . . . 18

2.2.4 X-ray photoelectron spectroscopy (XPS) . . . . . . . . . . . . 19

2.3 Tip preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.3.1 Tip fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.3.2 Tip cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

iv

Table of Contents 　

2.4 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Chapter 3 Atomic structures and local electronic states of clean

Fe3O4(001) surface 27

3.1 Surface termination and reconstruction . . . . . . . . . . . . . . . . . 27

3.1.1 B-layer surface termination . . . . . . . . . . . . . . . . . . . 29

3.1.2 (√2×

√2)R45◦ surface reconstruction . . . . . . . . . . . . . 30

3.2 Local electronic states of surface Fe atoms . . . . . . . . . . . . . . . 32

3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Chapter 4 Electronic structures of Fe3O4(001) subsurface 37

4.1 Subsurface structure models . . . . . . . . . . . . . . . . . . . . . . . 38

4.1.1 Charge-ordered structure . . . . . . . . . . . . . . . . . . . . . 38

4.1.2 Cation vacancy structure . . . . . . . . . . . . . . . . . . . . . 38

4.2 Local electronic states at narrow/wide section of surface reconstruction 39

4.3 Periodic spatial modulation of local density of states . . . . . . . . . 41

4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Chapter 5 Atomic structures of H/Fe3O4(001) surface 45

5.1 Surface OH groups on UHV-prepared Fe3O4(001) film . . . . . . . . . 45

5.2 Origin of surface OH groups . . . . . . . . . . . . . . . . . . . . . . . 49

5.3 Atomic structure relaxation by hydrogen adsorption . . . . . . . . . . 51

5.4 Adsorption-site dependences for H atoms . . . . . . . . . . . . . . . . 53

5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Chapter 6 Local electronic states of H/Fe3O4(001) surface 57

6.1 Effect of H atoms on the surface Fe electronic states . . . . . . . . . . 57

6.2 Correlation between OH density and surface Fe electronic states . . . 62

6.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Chapter 7 Summary 69

References 　 71

v

Table of Contents 　

Publication List 　 83

International Confererence　 85

Domestic Confererence　 87

vi

List of Figures

　

Figure 1.1 Concept of spintronics . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 1.2 Density of states of ferromagnetic materials (a) and half-metallic

materials (b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 2.1 Omicron UHV surface science system . . . . . . . . . . . . . . . . 9

Figure 2.2 Growth and measurement system of Fe3O4(001) film samples . . 10

Figure 2.3 Quantum mechanical tunneling when a voltage V is applied between

tip and sample. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 2.4 Schematic of STM system. . . . . . . . . . . . . . . . . . . . . . . 13

Figure 2.5 Quantum mechanical tunneling when a positive (a) or a negative

(b) bias voltage is applied to the sample. . . . . . . . . . . . . . . . 14

Figure 2.6 STM image of Si(111)(7×7) reconstructed surface. The polarity of

the sample bias voltage is changed from +1.5 (upper half) to –1.5 V

(lower half) in the middle of scanning over the surface. . . . . . . . 15

Figure 2.7 (a) STM image of Si(111)(7×7) surface. VS = –1.0 V, IT = 0.3 nA.

(b) dI/dV spectra obtained on three types of Si sites. . . . . . . . . 17

Figure 2.8 (a) STM image and (b) dI/dV image of Si(111)(7×7) surface. VS

= –1.0 V, IT = 0.3 nA. . . . . . . . . . . . . . . . . . . . . . . . . 19

vii

List of Figures 　

Figure 2.9 Schematic of XPS system. . . . . . . . . . . . . . . . . . . . . . . 20

Figure 2.10 XPS spectrum of an as-grown Fe3O4(001) film . . . . . . . . . . . 20

Figure 2.11 Schematic of tip etching system. . . . . . . . . . . . . . . . . . . 22

Figure 2.12 SEM image of the fabricated W tip. . . . . . . . . . . . . . . . . 23

Figure 2.13 STM images of Si(111)(7×7) surface obtained before tip cleaning

(VS = +2.0 V, IT = 1.0 nA) (a) and after tip cleaning (VS = +2.0

V, IT = 0.5 nA) (b). . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 2.14 RHEED patterns taken from the [100] direction. (a) MgO(001)

substrate after annealed in oxygen. (b) Fe3O4(001) films epitaxially

grown on MgO(001) substrate . . . . . . . . . . . . . . . . . . . . . 25

Figure 3.1 Cubic inverse spinel structure of Fe3O4. Tetrahedral iron in the A-

plane (FeA), octahedral iron (FeB) and oxygen atoms in the B-plane

are indicated by purple, red and gray circles. . . . . . . . . . . . . 28

Figure 3.2 (a) Overview STM image of Fe3O4(001) film surface. The feedback

control set point was VS = +2.0 V, IT = 1.0 nA. Atomically-flat

terraces exhibiting [110] or [1–10] atomic rows can be seen. (b) Line

profile taken along the black line in (a). The step height of ∼0.21 nm

is indicated. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 3.3 (a) High-resolution STM image (3.5 × 3.5 nm2) of Fe3O4(001) film

surface. The feedback control set point was VS = +1.0 V, IT = 0.3

nA. The (√2 ×

√2)R45◦ reconstructed unit cell is indicated by the

white square. The narrow and wide sections are marked as “n” and

“w”, respectively. (b) Line profiles corresponding to the lines AA and

AB in (a). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 3.4 Model of the B-terminated surface structure of Fe3O4(001). The

viii

List of Figures 　

black arrows indicate the directions of the displacements of octahedral

iron atoms of the top layer. The (√2×

√2)R45◦ reconstructed unit

cell is indicated by the black square. The narrow and wide sections

are marked as “n” and “w”, respectively. . . . . . . . . . . . . . . . 31

Figure 3.5 Normalized dI/dV spectrum numerically obtained from the I(V)

curves taken on surface FeB site situated at a terrace using the method

proposed by Feenstra [75]. The onsets of the occupied and unoccu-

pied states of the FeB atom were determined from linear extrapola-

tions, indicated by the dashed lines, and are marked as OO and UO,

respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 3.6 Normalized dI/dV spectrum numerically obtained from the I(V)

curves taken on surface FeB site situated at a step edge using the

method proposed by Feenstra [75]. The onsets of the occupied and

unoccupied states of the FeB atom were determined from linear ex-

trapolations, indicated by the dashed lines, and are marked as OO

and UO, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figure 4.1 Model of the subsurface charge-ordered structure (a) and the sub-

surface cation vacancy structure (b) of Fe3O4(001). The black arrows

indicate the directions of the displacements of FeB atoms of the top

layer. The (√2×

√2)R45◦ reconstructed unit cell is indicated by the

black square. The narrow and wide sections are marked as “n” and

“w”, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Figure 4.2 dI/dV spectra obtained at narrow and wide sections of the (√2 ×

√2)R45◦ surface reconstruction perpendicular to the FeB rows. These

spectra were obtained with a z-offset of 300 pm toward the surface

with respect to the original set point of +1.2 V and 0.3 nA. . . . . 40

Figure 4.3 (a), (b) Two-pass scan images (5.0× 5.0 nm2). First, an unoccupied-

ix

List of Figures 　

state STM image (a) was taken at a set point of +1.0 V and 0.3 nA.

Second, a dI/dV image (b) was taken at a sample bias voltage of –0.4

V. The topography recorded in (a) was played with a z-offset of 100

pm toward the surface. The narrow and wide sections are marked as

“n” and “w”, respectively. (c) Line profiles taken along the arrows

marked as X and Y in (a) and (b), respectively. . . . . . . . . . . . 42

Figure 5.1 Overview STM image (40 × 40 nm2) of the as-grown Fe3O4(001)

film surface. The feedback control set point was VS = +1.2 V, IT =

0.3 nA. The red and green ovals enclose regions containing Fe(C) and

some Fe(H) atoms, respectively. . . . . . . . . . . . . . . . . . . . . 46

Figure 5.2 Shirley background-subtracted O1s XPS spectrum (Mg Kα pho-

tons) from the Fe3O4(001) film surface. The spectrum is decomposed

into two components, as discussed in the text. . . . . . . . . . . . . 47

Figure 5.3 High-resolution STM image (3.0 × 3.0 nm2) of an area containing

both Fe(C) and Fe(H) atoms, with an overlaid atomic arrangement

model. The feedback control set point was VS = +1.2 V, IT = 0.3

nA. The white square represents the (√2 ×

√2)R45◦ unit cell. The

red, green, gray, and blue circles represent Fe(C), Fe(H), O, and H

atoms, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Figure 5.4 Variation of the number of BPs observed in 40 × 40 nm2 STM

images of a single sample surface as a function of the holding time at

UHV, tk. The straight line indicates an approximately linear relation. 50

Figure 5.5 (a) High-resolution STM image (3.5 × 4.5 nm2) of the area con-

taining both Fe(C) and Fe(H) atoms, and the atomic arrangement

models. The feedback control set point was VS = +1.2 V, IT = 0.3

nA. The white and yellow squares show the (√2×

√2)R45◦ and the

(1×1) symmetry, respectively. The white wavy and straight dashed

x

List of Figures 　

lines show Fe(C) and Fe(H) rows, respectively. The outside black ar-

row shows the line position of discontinuity for BPs. The inside black

arrow shows the direction of hydrogen hopping. (b) Cross-sectional

line profile taken along the white solid line shown in (a). The color

coding of the ions corresponds to the one in Fig. 5.3. . . . . . . . . 52

Figure 5.6 (a) High-resolution STM image (2.5 × 2.5 nm2) and the atomic ar-

rangement models. The feedback control set point was VS = +1.2 V,

IT = 0.3 nA. The white square shows the (√2×

√2)R45◦ symmetry.

(b) Overview STM image (40 × 40 nm2) of the as-grown Fe3O4(001)


0.3 nA. N1 and N2 or N3 and N4 in the inset show different hydrogen-

adsorption patterns for [110] or [1–10] atomic rows. N1 to N4 inside

the STM image correspond to their patterns. (c) Histogram showing

adsorption number counted from STM images for each adsorption-

site, i.e., N1 to N4. . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Figure 6.1 Normalized dI/dV spectra numerically obtained from the I(V) curves

taken on FeB sites using the Feenstra’s method [75]. The red and

green curves show the spectra obtained on Fe(C) and Fe(H) sites,

respectively. The result of Fe(C) site is equal to the one shown in

Fig. 3.5. The inset shows the region around Vs = 0, in which the

curves are offset vertically for clarity. The arrow in the inset denotes

the position of the small peak observed in the green curve. . . . . . 58

Figure 6.2 Schematic of multipass scanning method used in this study. (i)

At a sample bias voltage of +1.0 V, surface topography is measured

and recorded in the first pass using a constant-current mode. (ii) In

the second pass, at a sample bias voltage of –0.2 V, LDOS image

is obtained by detecting a tunneling current using the recorded tip

trajectory in the first pass. (iii) The procedure (i)–(ii) is performed

xi

List of Figures 　

in all lines of the scanning area. . . . . . . . . . . . . . . . . . . . . 59

Figure 6.3 (a), (b) Two-pass scan images (5.0× 5.0 nm2). First, an unoccupied-

state STM image (a) was taken at a set point of +1.0 V and 0.3 nA.

Second, a dI/dV image (b) was taken at a bias voltage of –0.2 V.

The topography recorded in (a) was played with a z-offset of 150 pm

toward the surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Figure 6.4 Schematic of electron transfer occurred in H/Fe3O4(001) surface.

The color coding of the ions corresponds to the one shown in Fig.

5.3. The arrows indicate the directions of electron flow. . . . . . . . 61

Figure 6.5 Overview STM image (40 × 40 nm2) of the as-grown Fe3O4(001)


0.3 nA. The yellow oval encloses a representative BP (SBP) which

corresponds to paired Fe(H) atoms. The yellow arrows show the

exceptional BPs (LBPs) whose length along the atomic rows is longer

than SBPs. The white circles enclose the SBPs adjacently arranged

perpendicular to the atomic rows. . . . . . . . . . . . . . . . . . . 62

Figure 6.6 (a) High-resolution STM image (2.4 × 1.6 nm2) of an area contain-

ing a SBP and the atomic arrangement model. (b) High-resolution

STM image (3.0 × 3.0 nm2) of an area containing one LBP and two

SBPs, and the atomic arrangement model. The feedback control set

point was VS = +1.2 V, IT = 0.3 nA. . . . . . . . . . . . . . . . . 63

Figure 6.7 (a) High-resolution STM image (3.0 × 3.0 nm2) of an area contain-

ing Fe(H1) and Fe(H2) atoms, and the atomic arrangement model.

The feedback control set point was VS = +1.2 V, IT = 0.3 nA. (b)

Normalized dI/dV spectra obtained on two FeB sites indicated in

panel (a) and on a Fe(C) site [66]. The normalized dI/dV spectra

were numerically obtained from the I(V) curves using the Feenstra’s

xii

List of Figures 　

method [75]. The inset shows the region around Vs = 0. . . . . . . 65

xiii

Chapter 1

Introduction

1.1 Background

1.1.1 Spintronics

Electronic devices have widely advanced since the invention of the first transistor

in 1947. The number of transistors can be doubled every two years. This well-known

law is called “Moore’s law”. The transistor size has also scaled down following this

law every year. As the transistor density has increased due to the shrinking in the

size, processing speeds have increased. As a result, information technology has been

rapidly advanced. Recently, however, following this Moore’s law is more difficult due

to various issues, such as high static power caused by the leakage currents [1,2]. The

static power arises mainly from the cache memory between the computing chip and

main memory. It prevents computing from reaching high frequency and limits power

efficiency, and thus the solutions to treat these issues are required. Development of

novel electronic devices to replace conventional charge-based complementary metal-

oxide-semiconductor circuits has attracted great attention.

Spintronics is an emerging field of electronics. This new discipline utilizes not

only the charge of the electrons but also their quantum property called “spin”,

as shown in Fig. 1.1. The Nobel Prize in Physics was awarded to Albert Fert

et al. in 2007 for their discovery of the giant magnetoresistance effect, which is

the cornerstone of spintronics. Since then, spintronic nanodevices, such as spin-

valve and tunnel magnetoresistance (TMR) devices, were rapidly applied in hard

disk drives. Nonvolatility is one of the most outstanding properties of spintronic

1

Chapter 1 Introduction

devices. Its property can power off the whole computing system in standby state.

For example, spin transfer torque magnetic random access memory (STT-MRAM)

can be a promising candidate for nonvolatile memories because of its fast speed and

high density [3]. In addition, the endurance of memory cells is very crucial for the

purpose of computing. Spintronics can provide nearly-infinite endurance to STT-

MRAM. Therefore, developing spin-based functionalities for the nonvolatile solid-

state memory and logic devices has been studied all over the world. A wide range

of studies on new fundamental physics and characterization methods in spintronics

have also been performed. These novel spintronic devices can realize extremely low

power consumption in the near future electronics.

SpintronicsLow-power consumption and high-performance device creation

Nonvolatile magnetic memory/sensor

Spin transistor Nonvolatile airthmetic circuit

Integration of electronics and magnetics

Electronics

Charge －

Magnetics

Spin

Electron

Data processing/computing Data recording/retention

Power consumption Volatile Magnetic storage (Nonvolatile)

－

Figure 1.1 Concept of spintronics

2


1.1.2 Half-metallic material

In recent years, there has been a rapid increase in the interest toward the ap-

plication of TMR devices with perpendicular magnetic anisotropy in spintronics,

such as an STT-MRAM. This interest is driven by the several promises of this

TMR device, such as high thermal and magnetic stabilities that will realize the

extremely low dimensional and high reliability devices in more advanced spintronic

applications. Until recently, TMR devices have been produced by sandwiching a

thin insulating layer with ferromagnetic electrode layers. However, to obtain higher

TMR values, more effective spin dependent scattering is required. Therefore, ferro-

magnetic materials with high electron-spin polarization are considered for spintronic

device applications. TMR values are expected to follow Julliere’s formula [4], TMR

= 2P1P2/(1–P1P2), where P1 and P2 are electron-spin polarizations of two ferro-

magnetic electrodes. Thus, finding and developing magnetic materials with higher

electron-spin polarization is one the most important factor for spintronic field. The

spin polarization is defined as the ratio of the density of states (DOS) of up-spin and

down-spin electrons at the Fermi level, P = (DOSup–DOSdown)/(DOSup+DOSdown).

In paramagnetic materials, for example, the DOS of up-spin and down-spin electrons

are equal, resulting in P = 0%. On the other hand, since the DOS of up-spin and

down-spins are different in ferromagnetic materials (see Fig. 1.2(a)), P values are

0–100%. The P values of Fe and Co are known to be about 50% [5]. If a material has

a semiconducting energy gap in the minority band at the Fermi level and exhibits

metallic behavior in the majority band, the DOS of the minority band is zero at

the Fermi level. In this case, only up-spin electrons are present at the Fermi level,

resulting in the P value of 100%. This type of material is called “half-metal” because

it shows both metallic and semiconducting behaviors, as shown in Fig. 1.2(b).

3


DOSDOS

EF

E

Up spin Down spin

EF

E

DOSDOS

Up spin Down spin

Energy gap

(a) (b)

Figure 1.2 Density of states of ferromagnetic materials (a) andhalf-metallic materials (b).

1.1.3 Characteristics of magnetite (Fe3O4)

Magnetite (Fe3O4) is a magnetic iron oxide that has the unusual properties such

as half-metallicity, high Curie temperature of 858 K and metal-insulator (Verwey)

transition at around 120 K in the bulk [6–9]. Moreover, Fe3O4 is abundantly present

in the earth and harmless to the human body. In the past decades, Fe3O4 has at-

tracted great attention because of its potential applications in spintronic devices,

catalyst, drug delivery and so on. Due to these outstanding properties, it is highly

desirable to grow high-quality Fe3O4 films. The film growth has been performed

using several deposition methods, such as molecular beam epitaxy [10–15], sputter-

ing [16] and pulsed laser deposition [17,18]. Many research groups have succeeded in

growing Fe3O4 films on various substrates (MgO, SrTiO3, MgAl2O4, sapphire, and

Si) [12,17,19–21]. However, these grown films have shown poor performances, such

as decreased electrical conductivity [12], unsaturated magnetization [16,22] and low

TMR values [23–26].

4


1.1.4 Disappearance of half-metallicity at the Fe3O4(001)

surface

As described above, the spin polarization value of electronic states is one of the

most significant indices for spintronic devices. Recently, there is considerable interest

in Fe3O4 due to its predicted half-metallic behavior in the bulk [6–8]. Consequently,

Fe3O4 films can be candidates for highly spin-polarized electrodes for spintronic

devices. However, the TMR values observed in Fe3O4-based devices [23–26] and

the spin injection efficiency obtained with a Fe3O4 electrode [27] have proved to

be much lower than expected. These poor performances are correlated with the

disruption of the half-metallic behavior at the material surface arising from the

surface states [28] and the antiphase domain boundary defects in Fe3O4 films [16,

29]. Spin-polarized photoelectron spectroscopy (SPPES) measurements of a clean

Fe3O4(001) surface have shown that the surface spin-polarization at the Fermi level

is –40% to –55% [30–32]. However, considering that the SPPES signal includes

a significant contribution from the bulk, these spin-polarization values should be

much lower at the surface. In fact, recent spin-polarized metastable de-excitation

spectroscopy (SPMDS) measurements, which can be more sensitive to the topmost

surface properties than SPPES, have demonstrated a less than –5% electron-spin

polarization at the Fermi level of a clean Fe3O4(001) surface [33–35]. To make

this material useful for spintronic applications, developing an effective method of

ensuring a surface electron-spin polarization close to that of the bulk is required.

5


1.1.5 Recovery of surface half-metallicity by hydrogen ad-

sorption

Recent experimental and theoretical studies have revealed that the adsorption of

hydrogen atoms on a clean Fe3O4(001) surface significantly enhances the surface

electron-spin polarization [28, 33–37]. SPMDS measurements have shown that the

spin polarization value at the Fermi level is enhanced by atomic-H adsorption to at

least –50% at room temperature [33–35]. Density functional theory (DFT) calcula-

tions have also predicted that half-metallic behavior can be achieved through atomic-

H adsorption [28,34–37]. In addition, the adsorption of benzene molecules [33], car-

bon [38] and boron atoms [39] has been found to enhance the surface electron-spin

polarization experimentally and theoretically. However, the detailed mechanism by

which the surface electron-spin polarization is affected has not been revealed. Al-

though theoretical predictions for the effect of hydrogen atoms on the surface local

electronic/spin states are reported [28, 35, 37], there are no experimental reports

of atomic-scale spectroscopic studies of an H-adsorbed Fe3O4(001) surface. More-

over, the modifications of the surface local electronic and electron-spin properties

induced by atomic-H adsorption have not been experimentally explored. However,

evaluation of the changes of surface electronic properties at the atomic level is very

crucial. This evaluation can advance our understanding of the interactions between

adsorbates and Fe3O4(001) topmost surface, and can contribute to an elucidation of

the origin of the enhanced electron-spin polarization of Fe3O4(001) surfaces.

6


1.2 Purpose

The broad purpose of this study is to elucidate the enhancement mechanism of

Fe3O4(001) surface electron-spin polarization. This mechanism can lead to avenue

for realizing a half-metallic electronic state of Fe3O4(001) surface and for inter-

face engineering in Fe3O4-based spintronic devices. The purpose of this work is

to reveal the effect of hydrogen atoms on the surface atomic geometries and local

electronic properties. To reveal these issues, atomic structures and iron electronic

states of a clean Fe3O4(001) surface were investigated by scanning tunneling mi-

croscopy/spectroscopy (STM/STS). Moreover, this work especially focused on mod-

ified iron atoms, whose electronic states are different from unmodified iron atoms by

surface OH groups. The modifications of atomic geometries by hydrogen adsorption

and the adsorption-site dependences for hydrogen atoms were investigated by STM

results. The differences in the local electronic states between unmodified and mod-

ified iron atoms were also investigated by STS results obtained at these two types

of iron sites. The origin of local electronic state modifications of surface iron atoms

was discussed from comparing the results of this experimental work and previous

theoretical ones. In addition, correlation between surface OH density and surface

iron electronic states was investigated by STM/STS.

This doctoral thesis is organized as follows.

In chapter 1, the research background and purpose of this study are described.

In chapter 2, the schematic of ultra-high vacuum (UHV) surface science system

used in this study, principles and measurement methods of STM/STS, and the

principle and schematic of X-ray photoelectron spectroscopy (XPS) are explained.

In addition, STM tip preparation and Fe3O4(001) film growth are described.

In chapter 3, atomic structures and local electronic states of a clean Fe3O4(001)

surface are discussed by STM/STS results. First, surface termination and surface

reconstructed structures are described using STM results. Next, the surface local

electronic states of terraces and step edges are discussed by energy-gap values of

7


surface iron atoms, which are determined from STS spectra.

In chapter 4, subsurface electronic structures are discussed using STS and dI/dV

mapping results. Periodic spatial modulation of local density of states was observed

in these measurements. These experimental results clearly demonstrate that the

subsurface FeB atoms show charge ordering of Fe2+-Fe2+ and Fe3+-Fe3+ dimers and

this charge ordering is responsible for the (√2×

√2)R45◦ surface reconstruction.

In chapter 5, atomic structures of H/Fe3O4(001) surface are mainly discussed

from STM results of the surface. First, the presence of surface OH groups of UHV-

prepared Fe3O4(001) films grown on MgO(001) substrates is verified by XPS. Next,

the origin of surface OH groups is investigated by STM measurements. Moreover,

changes of surface atomic structure by surface OH groups and adsorption-site de-

pendences for hydrogen atoms are discussed by STM results.

In chapter 6, local electronic states of H/Fe3O4(001) surfaces are discussed by

STM/STS and dI/dV mapping. First, effect of hydrogen atoms on the surface

iron electronic states are investigated by STS and dI/dV mapping. The electron-

transfer mechanism predicted to occur in this surface is discussed from comparing

these experimental and previous theoretical results. Moreover, correlation between

surface OH density and surface iron electronic states are discussed by atomic-scale

STM/STS.

In chapter 7, the main results of this doctoral thesis are summarized.

8

Chapter 2

Experimental setup

2.1 UHV surface science system

Scanning tunneling microscopy/spectroscopy (STM/STS) and X-ray photoelec-

tron spectroscopy (XPS) measurements were performed in an Omicron ultra-high

vacuum (UHV) surface science system shown in Fig. 2.1. The UHV system was

mainly composed of three chambers; load-lock chamber, preparation chamber and

analysis chamber. Sample and tip exchange was done through the load-lock cham-

ber. The vacuum of this chamber can be achieved below 5.0×10−7 Pa using a turbo

molecular pump (TMP), a titanium sublimation pump (TSP) and bake out system.

Loadlock Chamber

Analysis Chamber

Preparation Chamber

Figure 2.1 Omicron UHV surface science system

9

Chapter 2 Experimental setup

The preparation chamber was used for sample and tip preparation, equipped

with facilities such as four electron-beam heating evaporators (Omicron EFM3),

heating system of sample and tip, Ar ion sputtering system and reflection high

energy electron diffraction (RHEED) system. The vacuum of this chamber was kept

below 5.0× 10−9 Pa by a combination of TMP, sputter ion pump (SIP) and TSP.

The analysis chamber was used for sample preparation and characterization,

equipped with facilities such as one electron-beam heating evaporator (Omicron

EFM3), sample heating system, low energy electron diffraction system, XPS system,

Omicron UHV-STM system and Omicron variable temperature STM (VT-STM)

system. STM/STS and XPS experiments were performed in the analysis chamber.

STM/STS measurements were mainly done in the VT-STM system at room tem-

perature. The vacuum of analysis chamber was also kept below 5.0× 10−9 Pa by a

combination of TMP, SIP and TSP. The schematic of Fe3O4(001) film growth and

measurement system is depicted in Fig. 2.2.

Vacuum chamber

Heater

XPSsystem

STM/STSsystem

RHEEDscreen

RHEEDelectronbeam

MgO substrate

FeOxygen gas

Electron beamevaporator

Oxygen storage

Manipulator(Sample holder)

Leak valveNozzle

Preparation Chamber

Analysis Chamber

Figure 2.2 Growth and measurement system of Fe3O4(001) film samples

10


2.2 Experimental methods

Fe3O4(001) film samples were mainly characterized using STM, STS, differential

tunneling conductance (dI/dV) mapping and XPS. The detail of each experimental

method is explained below.

2.2.1 Scanning tunneling microscopy (STM)

STM was invented in the early 1980s by G. Binnig, H. Rohrer and co-workers at

the IBM Zurich research laboratory [40]. The Nobel Prize in Physics was awarded to

them in 1986 for their design of the STM. Since then it has become a fundamental

technique in surface science field due to its ability to probe material surfaces in

real space at the atomic scale. Among many experimental methods used for the

investigation of material surface physics, STM is one of the most powerful tools that

provide the geometric information of the surface with atomic resolution. The STM

can also perform the local tunneling spectroscopy called STS, which can provide the

density of states of atomic sites [41, 42], elementary excitation of phonon, magnon

[43], plasmon and so on. These two features of STM/STS technique enable us

to correlate the structural and electronic properties of a material surface at the

atomic resolution [44]. Therefore, STM/STS methods have been widely used for the

investigation of atomic-scale defects, adsorbed atoms/molecules [45, 46] and local

physics phenomena at the surfaces. Recently, spin-polarized STM/STS methods

using magnetic tips are used for the investigation of surface magnetism at the atomic

scale [47–50].

The STM principle is based on the quantum mechanical tunneling. In quantum

mechanics, electrons can tunnel through the vacuum barrier between two conductors

when positioned very close to each other, such as several nanometers. The tunneling

phenomenon is originated from the wavelike properties of electrons, as illustrated

in Fig. 2.3. Given that the vacuum barrier is one-dimensional, the solutions of the

Schrodinger equation inside the vacuum barrier have the following form,

11


sample vacuum tip

eV

ϕS

z

ϕT

EF

EF

VIT

Figure 2.3 Quantum mechanical tunneling when a voltage V isapplied between tip and sample.

ψ = e±κz (2.1)

When tip and sample are positioned very close, their electron wave functions can

overlap. The electron wave functions decay exponentially into the vacuum barrier

with the inverse decay length κ given by,

κ =

√2mϕ

ℏ(2.2)

where m is the electron mass, and the tunneling barrier height ϕ is defined by,

ϕ =ϕS

2+ϕT

2+eV

2− E (2.3)

When a voltage V is applied between tip and sample, the overlap of the electron

wave function leads to quantum mechanical tunneling and a current I will flow

across the vacuum barrier. The tunneling current I is given by,

12


I ∝ e−2κz (2.4)

where z is the tip-sample distance. The tunneling current depends exponentially

on the distance. If the distance is increased by 0.1 nm, the tunneling current is

decreased by one order of magnitude and vice-versa. This exponential dependence

of the tunneling current on the vacuum barrier width gives an unprecedentedly high

vertical resolution.

Figure 2.4 shows a schematic of STM system. The STM can be operated in

several modes. The most general measurement mode is the constant current mode.

A feedback control continuously adjusts the tip height by a piezoelectric scanner

to keep the constant current. By recording the voltage applied to the piezoelectric

driver as a function of lateral position in order to keep the tunneling current constant,

Tip trajectoryTip

Sample biasSample

XY

ZXYZ piezo-scanner

Tunnel current

Feedback circuit

High-voltageamplifier

Scan direction

Display

Figure 2.4 Schematic of STM system.

13


a topographic STM image can be acquired. The constant-current STM image is a

convolution of topographical and electronic state effects, which means that properly

speaking STM does not probe the atomic positions directly but the electron density

of states.

By changing the voltage polarity, both occupied and unoccupied electronic states

of the sample can be obtained with STM. When a positive voltage is applied to the

sample, electrons tunnel from occupied states of the tip to unoccupied states of the

sample. When a negative voltage is applied to the sample, electrons tunnel from

occupied states of the sample to unoccupied states of the tip. This electron flow is

illustrated in Fig. 2.5. Figure 2.6 shows an STM image of Si(111)(7× 7) surface, in

which the sample bias voltage is changed from +1.5 V (upper half) to –1.5 V (lower

half) in the middle of scanning. The appearance of STM image is greatly different

between the upper and the lower half. This difference is attributed to the access to

unoccupied and occupied states of the surface. Positive bias voltage gives mainly the

unoccupied states of adatoms (corner or center atoms), on the other hand negative

bias voltage contributes more to occupied states of rest atoms [51].

vacuum

ZZ

EF

EF

TunneleV

EF

EF

(a)

eV

DOS

(b)

Electron

vacuumvacuum

TipSampleTipSample

Tunnel

DOS

Figure 2.5 Quantum mechanical tunneling when a positive (a) or anegative (b) bias voltage is applied to the sample.

14


10 nm

positivebias voltage

negativebias voltage

Figure 2.6 STM image of Si(111)(7×7) reconstructed surface. Thepolarity of the sample bias voltage is changed from +1.5 (upper half)to –1.5 V (lower half) in the middle of scanning over the surface.

15


2.2.2 Scanning tunneling spectroscopy (STS)

The STM can obtain not only topographic information but also local electronic

density of states (LDOS) of a material surface using STS method. In this method,

the feedback loop is turned off and the tip-sample distance is kept constant. Then

a voltage sweep is performed and the tunneling current is recorded at each voltage

point. The differential of this tunneling current is proportional to the LDOS in the

particular point, see below. This spectroscopic signature of the LDOS gives a direct

information about energy dependences of the LDOS, such as surface energy gap

values.

The tunneling current I as a function of the bias voltage V applied between tip

and sample can be expressed by,

I =

∫ eV

0

ρs(E)ρt(−eV + E)T (z, eV, E)dE (2.5)

where ρs and ρt are DOS of sample and tip, respectively, and T is the transmission

probability [52]. Keeping the ρt constant and taking the first derivative of (2.5) with

respect to the bias voltage V ,

dI

dV∝ eρtρs(eV )T (z, eV, eV ) +

∫ eV

0

ρs(E)ρtdT (z, eV, E)

dVdE (2.6)

The constant ρt can be justified using a clean and metallic tip. The tunneling

current is dominated by a single s state of the tip [53]. The first term in equation

(2.6) is directly proportional to the sample DOS, and the second term contains

the voltage dependence of the transmission probability. The transmission factor T

monotonically increases with V and the contribution of the second term in (2.6) is a

smoothly varying background. Since the increase is smooth and monotonic, dI/dV

structures as a function of V can be assigned at an approximation to changes in the

sample DOS via the first term. As a result, equation (2.6) shows that dI/dV curves

give a direct information about the sample LDOS.

16


Figure 2.7(a) and (b) show a high-resolution STM image of Si(111)(7×7) surface

and representative dI/dV spectra obtained on three types of Si sites shown in Fig.

2.7(a). These dI/dV spectra show higher occupied LDOS (in particular, near –1.0

eV) of rest atoms and higher unoccupied LDOS of adatoms. This dI/dV feature is

reproduced by dI/dV mapping method, as described below.

1 nm 210-1-2

Sample Bias (V)

4

3

2

1

0

dI/d

V (

nS

)

corner atom

rest atom

center atom

(a) (b)

Figure 2.7 (a) STM image of Si(111)(7×7) surface. VS = –1.0 V, IT= 0.3 nA. (b) dI/dV spectra obtained on three types of Si sites.

17


2.2.3 Differential tunneling conductance (dI/dV) mapping

When STM users need to obtain only the spectroscopic contrast at a specific en-

ergy, dI/dV mapping method can be highly effective. This is a less time-consuming

method to get access to the electronic properties of the whole scanning area of

samples. In this method, the dI/dV signal at a specific bias voltage is recorded

at each point of the scanning area using standard lock-in technique with a mod-

ulation of bias voltage, simultaneously regular constant-current STM images are

obtained with closed feedback. Therefore, the topographic and the spectroscopic

data can be obtained simultaneously with high spatial resolution in a short time,

which permits a direct correlation of the topographic and the electronic properties

at the atomic scale. However, different bias voltages generally stabilize the tip at

different tip-sample distances, and thus comparing dI/dV images at different ener-

gies is so difficult. In this method, since the feedback loop is closed when recording

the dI/dV signal, the modulation should be much faster than the response of the

feedback loop so that a crosstalk between topographic image and dI/dV signal can

be avoided. Figure 2.8(a) and (b) show STM and dI/dV images of Si(111)(7 × 7)

surface obtained at a sample bias voltage of –1.0 V, respectively. By comparing

these two images, the rest atoms are observed to be more brighter than the adatoms

in the dI/dV image. Since the brightness in the dI/dV image reflects the LDOS

intensity at a specific energy, this result clearly shows much higher LDOS at –1.0 eV

of rest atoms than adatoms, which is in good agreement with the STS result shown

in Fig. 2.7(b). Moreover, it can be seen from these results that the LDOS at –1.0

eV of rest atoms of unfaulted half is higher than that of faulted half. In this way,

dI/dV mapping method is a powerful tool in surface science field due to the ability

to obtain the spatial distribution of sample LDOS at the atomic level.

18


(a)

1 nm 1 nm

(b)

unfaulted half

rest atom

faulted half unfaulted half faulted half

Figure 2.8 (a) STM image and (b) dI/dV image of Si(111)(7×7)surface. VS = –1.0 V, IT = 0.3 nA.

2.2.4 X-ray photoelectron spectroscopy (XPS)

XPS is one of the quantitative spectroscopic technique that measures the elemental

composition and chemical state of a material in its as-grown state or after some

surface treatments. XPS spectra are obtained by irradiating a material with a beam

of X-rays (see Fig. 2.9) while simultaneously measuring the kinetic energy and the

number of electrons that escape from the material surface at the depth of 1 to 5 nm.

By utilizing angle-resolved XPS technique, chemical states of the topmost surface

can also be obtained. Figure 2.10 shows a representative XPS spectrum of an as-

grown Fe3O4(001) film grown on the MgO(001) substrate. The spectrum shows

sharp peaks originated from Fe 2p and O 1s core levels, and shows that the grown

films are mainly composed of iron and oxygen elements. This XPS technique also

plays a significant role in investigating adsorbed atoms/molecules on the surface,

such as carbon-based impurities. In this work, the presence of OH groups on UHV-

prepared Fe3O4(001) film surfaces was verified by XPS, and thus this knowledge

obtained from this XPS technique became the basis of this study on H/Fe3O4(001)

surface physics.

19


X-ray

X-ray source

Sample

Charge neutraliser

Scan plates

Spotsize aperture

Electrostatic lens

Inner hemisphere

Outer hemisphere

UHV

Detector

Electrostatic hemisphere

Figure 2.9 Schematic of XPS system.

Binding Energy (eV)

Ph

oto

em

issio

n In

ten

sity (

a.u

.)

800 600 400 200 0

Fe 2p

O 1s

Figure 2.10 XPS spectrum of an as-grown Fe3O4(001) film

20


2.3 Tip preparation

2.3.1 Tip fabrication

In STM, tunneling current depends strongly on both tip and sample electronic

states. Therefore, the quality of a prepared STM tip is an important factor to obtain

highly reproducible STM images and reliable STS spectra. A variety of methods to

make a high-quality STM tip have ever been developed [45,54–56] and each method

has been refined [57–59]. Electrochemical etching has been widely used to make

a sharp STM tip. In this work, this general method was used to make a sharp

tip reproducibly. The chemical reaction process of this method is described below.

Polycrystalline tungsten (W) wire was used as a tip material. Figure 2.11 shows a

schematic of the tip etching system. As a first step, a W wire with a diameter of 0.30

mm was cut in ∼2.0 cm length. After that the wire was ultrasonically cleaned in

acetone for ten minutes. During etching, a W wire and a Pt wire behave as an anode

and a cathode, respectively. A NaOH solution (2 mol/l) was used as a electrolyte.

Applying the voltage between the anode and the cathode, the W wire was etched.

The following chemical reaction occurs during etching [60,61].

Anode W : W(s) + 8OH− → WO2−4 + 4H2O+ 6e− (2.7)

Cathode Au : 6H2O+ 6e− → 3H2(g) + 6OH− (2.8)

Whole response : W(s) + 2OH− + 2H2O → WO2−4 + 3H2(g) (2.9)

The etching reaction proceeds intensively at the interface between air and elec-

trolyte, and thus the neck is formed at the interface as depicted in Fig. 2.11. As

the etching reaction proceeds, the neck becomes too thin to sustain the weight of

the W wire in the solution. As a result, the W wire in the solution drops off. To

make a high-quality STM tip, keeping the stable interface of electrolyte without

vibration is vital. However, the H2 gas bubbles generated from the cathode usually

make an unstable interface. Therefore, I used a glass tube coiled by the Pt wire and

21


put the W wire into the glass tube, as shown in Fig. 2.11. This system avoids the

gas bubbles from disturbing the interface in the glass tube. It is also important to

cut off the etching current immediately after the etching, because the current can

make the tip blunt quickly [61]. For this reason, the differentiating circuit to cut

off the current was utilized. The etching current decreases suddenly at the instant

of the completing of the etching. The differentiator detects the sudden decrease in

the current and then the electronic circuit immediately stop the current. After the

etching, the etched W tip was rinsed in a hot distilled water to remove the residual

NaOH on the tip. The curvature radius of the fabricated tips is estimated to be

ranging from 20 to 50 nm. One of the scanning electron microscope (SEM) images

of the W tip is shown in Fig. 2.12.

W wire

Glass tube

Pt electrode

NaOH aq

Differentiating circuit

OH -OH -

WOH4

2- flow

W wire

Glass tube12 V

DC

power supply

Figure 2.11 Schematic of tip etching system.

22


100 μm

Figure 2.12 SEM image of the fabricated W tip.

2.3.2 Tip cleaning

Although a sharp STM tip can be made by the electrochemical etching method

described above, the atomic composition and arrangement at the tip apex are the

most important factor for electron tunneling process in STM. Therefore, the clean-

ing method to remove the nature oxide layer and contaminants at the tip apex is

essential. Two types of cleaning methods were adopted in this work. The first

cleaning method is electron bombardment heating method. In this method, a W

tip is positioned in front of a W filament heated by direct current. Applying the

voltage between the tip and the filament, thermal electrons are emitted from the

filament to the W tip. As a result, the W tip is heated and its apex is cleaned.

The second cleaning method is to scan a tip at high bias voltage. By scanning a tip

over a clean Si(111)(7 × 7) surface at a sample bias voltage of –10 V to –5 V, the

contaminants at the tip apex gradually fall down. An STM image of Si(111)(7×7)

surface obtained before tip cleaning is shown in Fig. 2.13(a), in which the con-

taminants are clearly seen from place to place. Combining these two methods, the

atomic-resolution STM images of Si(111)(7 × 7) surface could be reproducibly ob-

23


served, as shown in Fig. 2.13(b). In addition, the cleaned tip became more stable

by the bias voltage sweeping, which is very important for STS measurements. Since

the tip apex was unexpectedly and spontaneously contaminated during scanning

over sample surfaces, negative voltage pulses (–10 V to –5 V) were also frequently

performed for cleaning.

(a)

5 nm 5 nm

(b)

Figure 2.13 STM images of Si(111)(7×7) surface obtained beforetip cleaning (VS = +2.0 V, IT = 1.0 nA) (a) and after tip cleaning(VS = +2.0 V, IT = 0.5 nA) (b).

24


2.4 Sample preparation

The Fe3O4(001) film samples were epitaxially grown on MgO(001) single-crystal

substrates by electron-beam deposition of iron in the presence of oxygen [10,11,62–

67]. MgO(001) was used as the substrates because of its small lattice mismatch of ∼0.3%. A lattice constant of MgO and Fe3O4 is 0.4212 and 0.8397 nm, respectively. In

this work, mechanically-polished and cleaved MgO(001) substrates were used. After

organic solvent cleaning, the MgO(001) substrates were cleaned in UHV by heating

at 573 K over 16 hours and then annealed at 1073 K for 1 hour in oxygen atmosphere

(7.0×10−5 Pa). The RHEED pattern shown in Fig. 2.14(a) indicates the preparation

of a clean MgO(001) substrate surface. After that the iron deposition was performed

using an electron-beam heating evaporator at a substrate temperature of 523 K.

During the film formation, the oxygen pressure was set in the range from 7.0× 10−5

to 1.0×10−4 Pa. The growth rate was 0.9 ML/min and the film thickness was 20 nm.

After the deposition, the grown films were post-annealed at 523 K for 30 min in the

same oxygen atmosphere to obtain an atomically-flat surface. The RHEED pattern

shown in Fig. 2.14(b) indicates the well-known growth properties of Fe3O4(001)

films on MgO(001) substrates, showing a (√2×


(a) (b)

Figure 2.14 RHEED patterns taken from the [100] direction. (a)MgO(001) substrate after annealed in oxygen. (b) Fe3O4(001) filmsepitaxially grown on MgO(001) substrate

25


26

Chapter 3

Atomic structures and local

electronic states of clean

Fe3O4(001) surface

In this chapter, atomic structures and local electronic states of a clean Fe3O4(001)

surface are mainly discussed by scanning tunneling microscopy/spectroscopy (STM/STS)

results. First, surface termination and reconstruction are investigated by overview

and high-resolution STM images of the surface. Next, the surface local electronic

states are discussed by STS. Energy-gap values of surface iron atoms, which are

determined from STS spectra, are especially focused on. The difference in the elec-

tronic states between iron atoms of terraces and those of step edges is also discussed.

3.1 Surface termination and reconstruction

Bulk Fe3O4 has a cubic inverse spinel structure. In the [001] direction, A layers

of tetrahedral iron atoms (FeA) and B layers containing octahedral iron (FeB) and

oxygen atoms are stacked alternately as shown in Fig. 3.1. A B-terminated surface

with the (√2×

√2)R45◦ reconstruction has often been observed on the Fe3O4(001)

surface by STM [30,68, 69], and theoretical prediction also shows the energetically-

stable modified B-layer termination of Fe3O4(001) surface [70, 71]. Several models

have been proposed to explain the surface reconstruction on the basis of surface

charge ordering [69], the presence of oxygen vacancies on the surface [68,72], Jahn-

Teller distorted bulk truncation [70] or subsurface cation vacancy structure [73].

27

Chapter 3 Atomic structures and local electronic states of clean Fe3O4(001) surface

0.6 nm0.3 nm

B-plane

[010]

[100]A-plane

A-site

B-site0.8

4 n

m

A-A

B-B

0.21 nm

0.21 nm

[001]

[100]

[010]

A-B

0.105 nm

: Tetrahedral site Fe

: Oxygen atom

: Octahedral site Fe

Figure 3.1 Cubic inverse spinel structure of Fe3O4. Tetrahedraliron in the A-plane (FeA), octahedral iron (FeB) and oxygen atomsin the B-plane are indicated by purple, red and gray circles.

28


3.1.1 B-layer surface termination

An STM image of the Fe3O4(001) film surface is shown in Fig. 3.2(a). Atomically-

flat terraces can be seen in the STM image. The minimum step height is ∼0.21 nm,

as evident in the line profile in Fig. 3.2(b). This step height corresponds to the A-A

or B-B layer separation distance of Fe3O4(001). This STM result indicates that the

surface is terminated by the A-plane or the B-plane without the coexistence of both

layers. Moreover, a high-resolution STM image is shown in Fig. 3.3(a). As evident

in the line profile AA in Fig. 3.3(b), the average distance between neighboring atoms

within the rows is ∼0.3 nm. Since this periodicity of the atoms correspond to that

of the FeB atoms in the B-terminated Fe3O4(001) surface, as shown in Fig. 3.1, the

atoms observed in the STM images of Fe3O4(001) surface should represent single

FeB atoms in the B-plane.

Distance (nm)

00

0.1

0.2

~0.21 nm

~0.21 nm

0.3

0.4

0.5

0.6

2 4 6 8 10 12 14 16

He

igh

t (n

m)

10 nm

(a) (b)

[010]

[100]

Figure 3.2 (a) Overview STM image of Fe3O4(001) film surface.The feedback control set point was VS = +2.0 V, IT = 1.0 nA.Atomically-flat terraces exhibiting [110] or [1–10] atomic rows canbe seen. (b) Line profile taken along the black line in (a). The stepheight of ∼0.21 nm is indicated.

29


3.1.2 (√2×

√2)R45◦ surface reconstruction

As evident in the line profile AB in Fig. 3.3(b), the average distance between

neighboring rows is ∼0.6 nm. The STM image and line profile AB also indicate that

the atoms within each row are slightly shifted along the direction perpendicular to

the rows. This STM image and line profile clearly demonstrate the presence of nar-

row and wide sections, as depicted in the atomic arrangement model in Fig. 3.4.

The narrow and wide sections are marked as “n” and “w”, respectively. These small

displacements of the atoms perpendicular to the rows have been observed previously

in density functional theory (DFT) and low energy electron diffraction analysis stud-

ies [30, 31, 70, 71]. These STM results are consistent with a Jahn–Teller distorted

B-layer termination that is established to be energetically favorable over several oxy-

gen chemical potentials [70,71]. Surprisingly, recent studies of synthetic Fe3O4(001)

single crystal surfaces prepared by Ar ion sputtering followed by annealing in oxy-

gen have shown that the stable surface termination comprises subsurface FeB cation

He

igh

t (p

m)

[010]

[100]

(a) (b)

www

wwwnn

nn

AB

AA

Distance (nm)

40

30

20

10

02.52.01.51.00.50.0

~0.3 nmAA

120

80

40

02.52.01.51.00.50.0

AB

n w n

~0.6 nm

shift

Figure 3.3 (a) High-resolution STM image (3.5 × 3.5 nm2) ofFe3O4(001) film surface. The feedback control set point was VS =+1.0 V, IT = 0.3 nA. The (

√2 ×

√2)R45◦ reconstructed unit cell is

indicated by the white square. The narrow and wide sections aremarked as “n” and “w”, respectively. (b) Line profiles correspond-ing to the lines AA and AB in (a).

30


vacancies and interstitials [73]. However, this study assumes a Jahn–Teller distorted

B-layer termination without vacancies and interstitials because creating such vacan-

cies using atomic layer-by-layer molecular beam epitaxy should be difficult. This

distorted surface is stabilized by in-plane relaxations perpendicular to the atomic

rows, as illustrated in Fig. 3.4. Following a previous STM report of Fe3O4(001)

surface [74], I define the two nonequivalent surface oxygen atoms formed by the

distortion as ON and OW. The subscripts N and W included in these labels indicate

that the O–O distances at the surface are shorter and longer than that in the bulk,

respectively.

[010]

[100]

0.6 nm

n

ww

ww

0.3 nm

: Octahedral iron of top layer

: Oxygen of top layer

: Tetrahedral iron of subsurface layer

Figure 3.4 Model of the B-terminated surface structure ofFe3O4(001). The black arrows indicate the directions of the displace-ments of octahedral iron atoms of the top layer. The (

√2×

√2)R45◦

reconstructed unit cell is indicated by the black square. The narrowand wide sections are marked as “n” and “w”, respectively.

31


3.2 Local electronic states of surface Fe atoms

Local electronic states of surface FeB atoms are discussed here. To investigate

the local electronic states, I probed the local density of states (LDOS) of the FeB

atoms situated at terraces by STS. Figure 3.5 shows normalized dI/dV spectrum

numerically obtained from the I(V) curves taken on surface FeB site. In this study, I

followed the procedure proposed by Feenstra [75]. In this method, total conductance

(I/V) data are convoluted with an exponential function, relying on the broadening

width (∆V) to remove the background noise near Vs = 0. The convolution step

has no influence on the features of the spectra. In all cases ∆V = 0.6 V was used

according to the order of the energy-gap values of surface FeB atoms, which was

determined by STS spectra, as discussed below. Normalized dI/dV spectra used

here are found to provide a relatively direct measure of the surface LDOS [41, 52].

These methods also allow the detection of small LDOS near the Fermi level [52].

The STS spectrum in Fig. 3.5 shows pronounced increases for both Vs < 0 and Vs

> 0. The onsets of the occupied and unoccupied states were determined by applying

a linear extrapolation at the onset of the differential conductivity, as was previously

performed [76,77]. The onsets of the occupied and unoccupied states of the FeB atom

are observed at –0.44±0.01 and +0.24±0.01 eV, respectively. Thus, the energy-gap

value near the Fermi level of the surface FeB atom is estimated to be 0.68±0.01 eV.

An energy gap in the range 0.4–0.8 eV has been confirmed regardless of the tip and

sample used, and also on different FeB sites. The dispersion of the energy-gap values

may be attributed to tip-induced band bending (TIBB) effect, the difference in W

tip electronic states and a subtle influence from surrounding adsorbates. TIBB is

an effect where some of the applied bias decreases in the sample instead of in the

vacuum region, resulting in enlarged observed energy gaps [78,79]. Anyway, surface

FeB atoms of terraces show a semiconducting nature relative to the predicted half-

metallic state of bulk Fe3O4.

These observed energy-gap values of 0.4–0.8 eV are larger than the value of 0.2

eV reported in a previous STS study of a single crystalline Fe3O4(001) surface [80].

32


01.51.00.50.0−0.5−1.0−1.5

Sample Bias (V)

(dI/d

V)/

(I/V

) (a

.u.)

4

8

12

OO UO

~0.7 V

Figure 3.5 Normalized dI/dV spectrum numerically obtained fromthe I(V) curves taken on surface FeB site situated at a terrace usingthe method proposed by Feenstra [75]. The onsets of the occupiedand unoccupied states of the FeB atom were determined from linearextrapolations, indicated by the dashed lines, and are marked as OO

and UO, respectively.

The previously reported value was obtained from averaged I(V) curves taken over

a specific area of the Fe3O4(001) surface. Therefore, the value might include local

electronic states of surface atoms other than FeB atoms. The I(V) curves could also

be taken near step edges. Such areas have more oxygen vacancies [80] and more

adsorbates such as OH groups [65] than terraces, which can modify the surface local

electronic properties due to coordinative unsaturation and excess charges, which

could account for the observed changes in the I(V) characteristics. Indeed, the STS

spectrum measured at step edges shows a metallic-like behavior and the obtained

energy-gap values were 0.1-0.2 eV, as shown in Fig. 3.6.

33


10

8

6

4

2

01.00.50.0−0.5−1.0

Sample Bias (V)

(dI/dV)/(I/V) (a.u.)

OO UO

~0.2 V

Figure 3.6 Normalized dI/dV spectrum numerically obtained fromthe I(V) curves taken on surface FeB site situated at a step edgeusing the method proposed by Feenstra [75]. The onsets of theoccupied and unoccupied states of the FeB atom were determinedfrom linear extrapolations, indicated by the dashed lines, and aremarked as OO and UO, respectively.

The observation of an energy gap of 0.68 eV for the surface FeB atom of terraces

is consistent with the LDOS projected on the FeB 3d orbitals for the first surface

layer, which has been obtained by DFT calculations [81]. Judging from the DFT

results, occupied and unoccupied 3d states of surface FeB atoms are located below

–0.4 eV and above +0.2 eV, respectively, suggesting an about 0.6 eV energy gap

near the Fermi level. Since the surface FeB atom resides in an exclusively Fe3+-

like state due to the charge and orbital ordering of Fe2+ and Fe3+ in the subsurface

layers, which is confirmed by DFT [37,81–84] and angle-resolved X-ray photoelectron

spectroscopy [36, 84], the onset of the occupied states observed at –0.44 eV should

arise from the occupation of 3d states corresponding to Fe3+-like state.

34


3.3 Summary

Atomic structures and local electronic states of a clean Fe3O4(001) surface were

investigated by STM/STS measurements. The STM studies showed the well-known

B-terminated surface with a (√2 ×

√2)R45◦ reconstruction. The reconstructed

(distorted) surface is stabilized by in-plane relaxations perpendicular to the atomic

rows. The atomic-scale STS studies revealed that the surface FeB atoms of terraces

show energy-gap values of 0.4–0.8 eV near the Fermi level. This STS result indicates

a semiconducting nature relative to the predicted half-metallicity of bulk Fe3O4.

Moreover, surface local electronic states of step edges were found to be metallic-like

with respect to a semiconducting nature of terraces, which can be due to the presence

of more oxygen vacancies and more adsorbed atoms/molecules at step edges.

35


36

Chapter 4

Electronic structures of

Fe3O4(001) subsurface

The Fe3O4(001) surface structure has been mainly studied by scanning tunneling

microscopy (STM) and low energy electron diffraction [30, 31, 68]. Several research

groups have observed individual FeB atoms and a (√2×

√2)R45◦ surface reconstruc-

tion using STM, which revealed wave-like structures of the FeB rows, as depicted in

Fig. 4.1(a) [30, 31, 62–66, 68]. Pentcheva et al. predicted the B-terminated surface

structure on the basis of density functional theory (DFT) calculations [70,71]. They

also suggested that the (√2×

√2)R45◦ surface reconstruction is a result of a Jahn–

Teller distortion and that the surface shows a metallic nature. However, Jordan et

al. reported a surface energy gap of ∼0.2 eV on the basis of a scanning tunnel-

ing spectroscopy (STS) study [80], which indicates a semiconducting nature of the

surface. In addition, this study shows the energy-gap value near the Fermi level of

the surface FeB atoms to be ∼0.7 eV. The reconstructed structure and the semicon-

ducting nature of the surface have been proposed to be derived from the subsurface

charge-ordered structures [81, 85, 86] or subsurface cation vacancy structures [73].

As described in the prior chapter, this study assumes a perfect Fe3O4 stoichiome-

try without cation vacancies, and thus the subsurface charge-ordered structures are

focused on and are discussed by STM/STS results in this chapter.

37

Chapter 4 Electronic structures of Fe3O4(001) subsurface

4.1 Subsurface structure models

Two types of subsurface structure models, which are charge-ordered structure

and cation vacancy structure, have been proposed to explain the (√2 ×

√2)R45◦

reconstruction of Fe3O4(001) surface and its semiconducting nature. The detail of

these two structure models is explained below.

4.1.1 Charge-ordered structure

Pentcheva et al. studied the Fe3O4(001) surface using a variety of methods and

proposed that the (√2×

√2)R45◦ surface reconstruction results from a lattice distor-

tion [70]. After that Lodziana proposed that the reconstruction could be interpreted

in terms of a surface Verwey transition using DFT calculations that account for the

on-site Coulomb interaction of Fe d electrons, i.e., subsurface charge ordering is re-

sponsible for the (√2×

√2)R45◦ surface reconstruction [81]. The proposed charge

ordering structure shown in Fig. 4.1(a) opens a small band gap of 0.2 eV in DFT

calculations, which is consistent with the STS results performed by Jordan et al [80].

4.1.2 Cation vacancy structure

The subsurface cation vacancy structure model of the (√2×

√2)R45◦ reconstruc-

tion was proposed in 2014 [73]. In this model, the surface FeB-O layer remains

stoichiometric, however is distorted by a rearrangement of the iron cations in the

subsurface layers. Especially, an additional interstitial tetrahedral Fe atom (Feint) in

the subsurface layer replaces two subsurface octahedral FeB atoms (see Fig. 4.1(b)),

resulting in a net reduction of one iron atom per unit cell. The presence of Feint in

the subsurface layer is predicted to block adsorption of metal adatoms. The DFT

calculations also indicate that the subsurface cation vacancy structure is thermody-

namically more stable than the previously proposed structures over the entire range

of oxygen chemical potentials accessible in ultra-high vacuum [73]. The outermost

38


unit cell has an intermediate stoichiometry, Fe11O16 and the DFT calculations pre-

dict that the outermost unit cell contains Fe3+ cations only [73], which is consistent

with angle-resolved X-ray photoelectron spectroscopy measurements [36,84].

[010]

[100]

: Octahedral iron (Fe2+) of subsurface layer



n

ww

ww

[010]

[100]

: Interstitial tetrahedral iron of subsurface layer


n

ww

ww


(a) (b)

Figure 4.1 Model of the subsurface charge-ordered structure (a)and the subsurface cation vacancy structure (b) of Fe3O4(001). Theblack arrows indicate the directions of the displacements of FeBatoms of the top layer. The (

√2 ×

√2)R45◦ reconstructed unit cell

is indicated by the black square. The narrow and wide sections aremarked as “n” and “w”, respectively.

4.2 Local electronic states at narrow/wide section

of surface reconstruction

We performed STS measurements on the Fe3O4(001) film surface to explore the

subsurface electronic structures. Figure 4.2 shows two dI/dV spectra obtained at

the previously described narrow and wide sections. These STS spectra were obtained

with a z-offset of 300 pm toward the surface with respect to the original set point

of +1.2 V and 0.3 nA so that the subsurface electronic states could be effectively

detected. This STS result shows the higher local density of states (LDOS) just

39


below the Fermi level of the narrow section relative to the wide one. These dI/dV

signals might include contributions from the surface FeB atoms, however all surface

FeB atoms exclusively exhibited an Fe3+ charge state [36, 66, 84–86]. If the spectra

included mainly the surface FeB LDOS, such a visible difference in the LDOS would

not be observed. Therefore, the dI/dV spectra in Fig. 4.2 should include mainly

the subsurface electronic states. The difference in the electronic states between the

narrow and the wide sections is also discussed by dI/dV mapping results in the next

section.

3

2

1

0

dI/

dV

(a

.u.)

−0.8 −0.6 −0.4 −0.2 0.0 0.2 0.4

Sample Bias (V)

Wide section

Narrow section

Figure 4.2 dI/dV spectra obtained at narrow and wide sections ofthe (

√2 ×

√2)R45◦ surface reconstruction perpendicular to the FeB

rows. These spectra were obtained with a z-offset of 300 pm towardthe surface with respect to the original set point of +1.2 V and 0.3nA.

40


4.3 Periodic spatial modulation of local density of

states

We also performed dI/dV mapping of the Fe3O4(001) film surface to further

elucidate the subsurface electronic structure. Figure 4.3(a) and (b) display a set of

images acquired using the multipass scanning method described in chapter 6.1 [87].

The STM (Fig. 4.3(a)) and dI/dV (Fig. 4.3(b)) images were obtained at a sample

bias voltage of +1.0 V and –0.4 V, respectively. Figure 4.3(c) shows cross-sectional

line profiles of the STM and dI/dV images taken along the arrows marked as X

and Y, respectively. The relative positions of the arrows are the same. In the STM

image, the FeB rows are running along the [110] direction. However, the dI/dV

image shows a periodical electronic structure that differs from that in the STM

image. It comprises two different dI/dV peak intensities (high peak and low peak),

as indicated by the line profile Y in Fig. 4.3(c). The line profile was taken along

the [1-10] direction, i.e., the direction perpendicular to the FeB rows. This result

indicates that the periodicities of high(low)-peak to high(low)-peak and high(low)-

peak to low(high)-peak are ∼1.2 and ∼0.6 nm, respectively.

In the dI/dV image shown in Fig. 4.3(b), periodical patterns with a (√2 ×

√2)R45◦ symmetry are clearly observed, as indicated by the white square. Two

types of bright spots in the dI/dV image are observed between the FeB rows (see

Fig. 4.3(a) and (b)). As evident in Fig. 4.3(c), the positions of high(low) dI/dV

peaks correspond to those of the narrow(wide) section, which is in good agreement

with the dI/dV spectra shown in Fig. 4.2. The observed dI/dV peaks are derived

from the electronic states of surface oxygen atoms or subsurface FeB atoms according

to the structure model shown in Fig. 4.1. Considering that DFT calculations have

predicted that the LDOS of surface oxygen atoms is almost zero near the Fermi level

[85] and that the LDOS at –0.4 eV of subsurface Fe2+ cations is much higher than

that of subsurface Fe3+ cations [81,85], the high(low) dI/dV peak should indicate the

presence of Fe2+-Fe2+(Fe3+-Fe3+) dimers in the subsurface layer. These experimental

findings clearly demonstrate the presence of subsurface charge ordering, and the

41


(a)

(b)

(c)60

40

20

0

He

igh

t (p

m)

43210

wn n w

X ~0.6 nm

8

6

4

2

0

dI/

dV

(a

.u.)

43210

Distance (nm)

Y

~0.6 nm

~1.2 nm

n

w

w

X

Y

wn

w

w

wn

[010]

[100]

[010]

[100]

Vs = 1.0 V

Vs = −0.4 V

Figure 4.3 (a), (b) Two-pass scan images (5.0 × 5.0 nm2). First, anunoccupied-state STM image (a) was taken at a set point of +1.0 Vand 0.3 nA. Second, a dI/dV image (b) was taken at a sample biasvoltage of –0.4 V. The topography recorded in (a) was played with az-offset of 100 pm toward the surface. The narrow and wide sectionsare marked as “n” and “w”, respectively. (c) Line profiles takenalong the arrows marked as X and Y in (a) and (b), respectively.

subsurface charge-ordered electronic structure is sufficient to explain the (√2 ×

√2)R45◦ surface reconstruction [81]. Moreover, these results indicate that a degree

of electronic ordering on the Fe3O4(001) surface observed in previously reported

STM images [62,69] likely corresponds to that on the subsurface, as discussed in this

doctoral thesis. Although the imaging mechanism of the subsurface electronic states

has not been clarified completely, we suggest that the subsurface FeB atoms can be

imaged on the basis of the insulating-like nature of the topmost Fe3O4(001) surface,

on which the energy gap of ∼0.7 eV is present near the Fermi level [66]. Theoretical

studies that directly compare the surface electronic states with the subsurface ones

are required to further elucidate the imaging mechanism.

42


4.4 Summary

STM/STS measurements of Fe3O4(001) film surfaces were performed to inves-

tigate the surface atomic structures and the subsurface electronic structures. In-

dividual FeB atoms and a B-terminated surface structure with a (√2 ×

√2)R45◦

reconstruction were observed by STM, as mainly described in the prior chapter.

The observed surface structure corresponds to the theoretical model reported by

Pentcheva et al. [70, 71] and to previous STM results [30, 31, 68]. The obtained

STS spectra and dI/dV image demonstrated the presence of charge ordering in the

Fe3O4(001) subsurface layer. The charge-ordered electronic structure with two al-

ternating types of LDOS exhibits a (√2 ×

√2)R45◦ symmetry and is consistent

with the structure model of subsurface charge ordering [81,85]. These experimental

findings clearly demonstrate that the subsurface FeB atoms show charge ordering

of Fe2+-Fe2+ and Fe3+-Fe3+ dimers and this charge ordering is responsible for the

(√2×


43


44

Chapter 5

Atomic structures of

H/Fe3O4(001) surface

Scanning tunneling microscopy (STM) studies of H/Fe3O4(001) surface are mainly

discussed in this chapter. First, the presence of surface OH groups on ultra-high

vacuum (UHV)-prepared Fe3O4(001) film is verified by X-ray photoelectron spec-

troscopy (XPS) measurements. Next, the origin of surface OH groups is investigated

by STM measurements. Moreover, changes of surface atomic structure by surface

OH groups and adsorption-site dependences for hydrogen atoms are discussed by

STM results.

5.1 Surface OH groups on UHV-prepared Fe3O4(001)

film

An overview STM image of the as-grown Fe3O4(001) film surface is shown in

Fig. 5.1. This result represents the formation of an atomically flat surface. STM

images of the unoccupied surface states show FeB atomic rows running along the

[110] or [1–10] direction [64–67]. Here, several bright protrusions (BPs) situated at

FeB sites are clearly observed within the atomic rows. The area enclosed by the

green oval contains some BPs, whereas that enclosed by the red oval contains none.

This difference in the apparent height indicates that the local electronic states of the

FeB atoms are significantly modified. Since STM images alone cannot explain the

reason for the observed modified electronic states, the surface chemical states were

45

Chapter 5 Atomic structures of H/Fe3O4(001) surface

[010]

[100]

Figure 5.1 Overview STM image (40 × 40 nm2) of the as-grownFe3O4(001) film surface. The feedback control set point was VS =+1.2 V, IT = 0.3 nA. The red and green ovals enclose regions con-taining Fe(C) and some Fe(H) atoms, respectively.

investigated by XPS in detail [66]. The obtained XPS spectra showed two distinct

features. A small peak at a binding energy (BE) of 284.6 eV and a slight asymmetry

in the O1s peak representing tailing toward the high BE side were observed. The

small peak at 284.6 eV is attributed to nonoxygenated carbon on the surface, as

previously reported [88]. The surface atomic concentration of C was estimated to

be ∼0.5%. Since many BPs were observed in the large-scale STM images as shown

in Fig. 5.1, the BPs cannot be attributed to carbon-based impurities. Figure 5.2

shows the Shirley background-subtracted O1s XPS spectrum that is fitted with two

components using mixed Lorentzian-Gaussian curves. The first and more intense

component peak located at 530.0 eV is assigned to the oxygen in Fe3O4 [88]. The

46


528529530531532533534

Binding energy (eV)

Inte

nsity (

a.u

.)

O in Fe3O4

BE = 530.0 eV

O in OHBE = 531.5 eV

527

Sum of the fitted curves

Figure 5.2 Shirley background-subtracted O1s XPS spectrum (MgKα photons) from the Fe3O4(001) film surface. The spectrum isdecomposed into two components, as discussed in the text.

other small component peak at 531.5 eV is strictly attributed to the oxygen in the

OH group [89], because the concentration of carbonyl groups, whose oxygen BE

resides in the same energy region, are negligible due to the low concentration of C

mentioned above. Therefore, the fitting results indicate that OH groups are present

on the UHV-prepared Fe3O4(001) film surface [66].

Henceforth, I refer to the BPs observed in the STM images as Fe(H) and, oth-

erwise, as Fe(C), which are FeB atoms residing in a clean (Jahn–Teller distorted)

B-layer surface. Figure 5.3 shows a high-resolution STM image of an area contain-

ing both Fe(C) and Fe(H) atoms, in which two adjacent FeB atoms are observed as

BPs. The previous STM studies of atomic-H [36] or disassociated-H2O [74] adsorbed

Fe3O4(001) surfaces performed by Parkinson et al. have revealed that surface OH

47


groups, which are formed by hydrogen bonding to ON sites, modify the local elec-

tronic states in the neighboring two FeB atomic pairs, as depicted by the atomic

arrangement model shown in Fig. 5.3. Therefore, the BPs observed in the STM

images of UHV-prepared Fe3O4(001) film surfaces can be attributed to surface OH

groups. The typical H coverage of the as-grown Fe3O4(001) film surface is 0.12 ML,

where 1 ML = 1 adsorbate per (√2 ×

√2)R45◦ unit cell, based on the proposed

model for atomic-H adsorption.

N

N

W

W

Fe(H) O HFe(C)

[010]

[100]

Figure 5.3 High-resolution STM image (3.0 × 3.0 nm2) of an areacontaining both Fe(C) and Fe(H) atoms, with an overlaid atomicarrangement model. The feedback control set point was VS = +1.2V, IT = 0.3 nA. The white square represents the (

√2×

√2)R45◦ unit

cell. The red, green, gray, and blue circles represent Fe(C), Fe(H),O, and H atoms, respectively.

48


5.2 Origin of surface OH groups

The origin of surface OH groups was explored by STM measurements. Figure 5.4

shows the variation of the number of BPs observed in 40 × 40 nm2 STM images of

a single sample surface as a function of the holding time at UHV, tk. In this work,

BPs observed in the proximity of surface defects such as step edges, and those larger

bright cluster-like protrusions are not counted because these protrusions might be

oxygen vacancies where extra electrons are trapped or carbon impurities originating

from UHV residual gases. This results show an approximately linear increase in

the number of BPs over time, indicating that surface OH groups are derived from

UHV residual gases. This tendency was also observed in other samples. In fact,

the presence of both cations and anions on metal oxide surfaces tends to promote

the dissociative adsorption of H2O [82, 83]. Thermodynamic calculations of the

Fe3O4(001) surface have also demonstrated the high stability of surface OH groups

in an UHV environment [37]. Therefore, the surface OH groups can be speculated

to arise from dissociative adsorption of UHV residual gases such as H2O and H2. In

this regard note that from Fig. 5.4, the number of BPs at tk = 0 is estimated to be

150. This indicates the presence of OH groups on the surface, even prior to the onset

of STM measurements, which may stem from the lower vacuum of the UHV transfer

system connecting the preparation and the analysis chambers, or from impurities

contained in the introduced oxygen gas. In addition, I followed the variation of

the O1s XPS spectrum as a function of tk, however visible changes showing the

increasing surface OH groups with tk were not observed. This nonobservation of

the changes can be due to the XPS detection limit including also the bulk chemical

structures.

49


100

150

200

250

0 50 100 150 200

Holding time at UHV, tk (hour)

Nu

mb

er

of p

rotr

usio

ns p

er

40

× 4

0 n

m2

Figure 5.4 Variation of the number of BPs observed in 40 × 40 nm2

STM images of a single sample surface as a function of the holdingtime at UHV, tk. The straight line indicates an approximately linearrelation.

50


5.3 Atomic structure relaxation by hydrogen ad-

sorption

Figure 5.5(a) also shows a high-resolution STM image of the area containing

both Fe(C) and Fe(H) atoms. It clearly shows that the configuration of Fe(C)

rows is wavy, on the other hand the Fe(H) rows are straight. Density functional

theory (DFT) calculations and STM experiments have demonstrated that FeB rows

of a clean Fe3O4(001) surface show wave-like structures [30, 31, 69–71]. However,

STM measurements of an H-adsorbed Fe3O4(001) surface have revealed that the

configuration of surface atomic rows turns to straight by hydrogen adsorption [36].

This STM result shown in Fig. 5.5(a) corresponds to the previous reports. Figure

5.5(b) shows a cross-sectional line profile taken along the white solid line shown

in Fig. 5.5(a). It shows not only the distances between two Fe(C) atoms along

the perpendicular direction to surface atomic rows but also the distances between

two Fe(H) atoms along the direction. The former distances consist of two different

values, 0.56 nm and 0.64 nm. Wavy rows observed in STM images are attributed

to such in-plane relaxations perpendicular to atomic rows direction for surface FeB

atoms, as described in chapter 3.1. On the other hand, the latter distances have a

single value, 0.60 nm. This value corresponds to the distance between two FeB atoms

along the direction in the bulk, indicating that atomic positions of Fe(H) atoms are

consistent with ones of bulk FeB atoms. Furthermore, it demonstrates that the

configuration of Fe(H) rows is straight similar to the bulk FeB rows, because atomic

displacements induced by OH groups are expected to occur at two surface FeB atoms

adjacent to them. This STM result showing adsorbed-H induced changes of surface

atomic geometries agrees well with the previous DFT results [36], which have shown

that the surface atoms relax back to bulk-terminated positions following hydrogen

adsorption. A clean Fe3O4(001) surface has the (√2 ×

√2)R45◦ reconstruction as

shown in Fig. 5.5(a), however low energy electron diffraction and DFT studies of

an H-adsorbed Fe3O4(001) surface have revealed that the surface exhibits (1×1)

symmetry same as the bulk [28,33,35,36].

51


43210

200

150

100

50

0

(b)

Distance (nm)

He

igh

t (p

m)

0.60 nm

0.56 nm 0.64 nm

0.60 nm(a)(√2×√2)R45˚

N

N

W

W

straight

wavy

(1×1)

[010]

[100]

Figure 5.5 (a) High-resolution STM image (3.5 × 4.5 nm2) of thearea containing both Fe(C) and Fe(H) atoms, and the atomic ar-rangement models. The feedback control set point was VS = +1.2V, IT = 0.3 nA. The white and yellow squares show the (

√2×

√2)R45◦

and the (1×1) symmetry, respectively. The white wavy and straightdashed lines show Fe(C) and Fe(H) rows, respectively. The outsideblack arrow shows the line position of discontinuity for BPs. Theinside black arrow shows the direction of hydrogen hopping. (b)Cross-sectional line profile taken along the white solid line shown in(a). The color coding of the ions corresponds to the one in Fig. 5.3.

An interesting phenomenon has often been observed during scanning over the

surface such that the BPs jump into a neighboring row as shown in the bottom

part of Fig. 5.5(a). The STM image discontinuously changes at the line position

indicated by the outside black arrow. In the STM image, the tip scanned the

surface from the top to the bottom and the left to the right. This type of switching

phenomenon of BPs has also been reported by the previous STM studies of an

H-adsorbed Fe3O4(001) surface [36]. The hydrogen atom adsorbed on an ON site

tends to hop into the neighboring ON site. In this STM measurement, the same

things were happened as indicated by the atomic arrangement model shown in Fig.

5.5(a). This hopping phenomenon did not depend on the scanning direction and

was frequently observed. I could identify more than one hopped site in nearly every

measurement (wide area scan: 40 × 40 nm2 to 50 × 50 nm2). These facts indicate

that hydrogen atom should be bonded to an ON site not an OW site, and hops into

a neighboring ON site rather than a neighboring OW site.

52


5.4 Adsorption-site dependences for H atoms

Recent DFT study of Fe3O4(001) surfaces with one hydrogen atom per unit cell

showed that the neighboring ON sites are not equivalent for hydrogen adsorption [37].

According to the DFT study, adsorption energies for these ON sites are different by

0.05 eV. To distinguish these two types of ON sites I label them as ON1 and ON2.

In the surface atomic arrangement model depicted in Fig. 5.6(a), these oxygen sites

within [110] atomic rows are marked as “N1”and “N2”, respectively. Since the DFT

result indicates that one of these sites is preferable for hydrogen to be adsorbed, I

have expected that STM images should reflect this difference. As seen in the high-

resolution STM image shown in Fig. 5.6(a), the H-adsorbed ON1 and ON2 sites are

both identified. Therefore, I investigated the numbers of H-adsorbed sites for their

ON atoms using larger-scale STM images (40 × 40 nm2 to 50 × 50 nm2) such as

Fig. 5.6(b). Checking more than 1000 BPs within [110] atomic rows observed in

the STM images of single sample surface, the histogram for the emergence at their

ON sites was obtained. The histogram shown in Fig. 5.6(c) suggests that there is

no obvious difference for the emergence between these sites.

I also performed the same investigations concerning two types of ON sites within

[1–10] atomic rows. Although each of these ON sites is equivalent to either ON1

site or ON2 site, the correspondence is unclear due to antiphase domain boundaries

defects introduced in the Fe3O4(001) films [64]. Therefore, I label them as ON3 and

ON4 (not ON1 and ON2). In the surface atomic arrangement model depicted in the

inset of Fig. 5.6(b), these oxygen sites are marked as “N3”and “N4”, respectively.

The histogram for the emergence at their ON sites shows the similar results to

the former. These experimental results show no clear adsorption-site dependences

between ON sites. Thus, I revealed that two types of ON sites of a Fe3O4(001)

surface are almost equivalent for hydrogen adsorption. Although statistical analyses

of hydrogen hopping frequency are not yet sufficient, there seems no clear difference

in the hopping between N1 → N2 and N2 → N1 or N3 → N4 and N4 → N3. The

hydrogen coverage on the as-grown Fe3O4(001) films surface is estimated to be 0.12

53


N1

N2

N1

N2

(a)

0

100

200

300

400

500

600

N1 N2

Counts

N3 N4

(c)

(b)

[010]

[100]

N4

N3

N3

N4

N3

N4

N1

N2

[010]

[100]

N1

N2

N1

N2

N1N2

N3N4

[010]

[100]

Figure 5.6 (a) High-resolution STM image (2.5 × 2.5 nm2) andthe atomic arrangement models. The feedback control set pointwas VS = +1.2 V, IT = 0.3 nA. The white square shows the (

√2 ×√

2)R45◦ symmetry. (b) Overview STM image (40 × 40 nm2) of theas-grown Fe3O4(001) film surface. The feedback control set pointwas VS = +1.2 V, IT = 0.3 nA. N1 and N2 or N3 and N4 in theinset show different hydrogen-adsorption patterns for [110] or [1–10]atomic rows. N1 to N4 inside the STM image correspond to theirpatterns. (c) Histogram showing adsorption number counted fromSTM images for each adsorption-site, i.e., N1 to N4.

ML, i.e., 12% of the ON sites are occupied. This study shows the priorities of H-

adsorbed sites at low hydrogen coverage, however there have also been no reports

on relations between hydrogen coverage and adsorption-site dependence. Moreover,

these results obtained at room temperature are somewhat different from the previous

theoretical prediction [37]. Further STM investigations of Fe3O4(001) surfaces with

varying hydrogen coverages and at varying temperatures are required to give newer

experimental insights into an H-adsorbed Fe3O4(001) surface.

54


5.5 Summary

Atomic structures of H/Fe3O4(001) surfaces were mainly investigated by STM

measurements. It was demonstrated using XPS and STM that the bright protru-

sions observed in the STM images of UHV-prepared Fe3O4(001) film surfaces can

be attributed to surface OH groups. STM studies revealed that the number of OH

groups increase with increasing UHV holding time of sample, which indicates that

the OH groups are derived from UHV residual gases. The STM studies also showed

that surface FeB atoms relax toward the bulk-terminated positions following hydro-

gen adsorption. I also investigated adsorption-site dependences for hydrogen atoms,

and revealed that there is no obvious dependence between two types of ON sites

of a Jahn-Teller distorted Fe3O4(001) surface, which is different from the previous

theoretical prediction.

55


56

Chapter 6

Local electronic states of

H/Fe3O4(001) surface

In this chapter, local electronic states of H/Fe3O4(001) surfaces are mainly discussed

by scanning tunneling microscopy/spectroscopy (STM/STS) results. First, effect of

hydrogen atoms on the surface iron electronic states are investigated by STS and

dI/dVmapping. The electron-transfer phenomenon predicted to occur in this surface

is discussed from comparing these experimental and previous theoretical results.

Moreover, correlation between surface OH density and surface iron electronic states

are discussed by atomic-scale STM/STS results.

6.1 Effect of H atoms on the surface Fe electronic

states

To investigate the effect of adsorbed H atoms on the local electronic states of

surface FeB atoms, I probed the local density of states (LDOS) of unmodified iron

(Fe(C)) and modified iron (Fe(H)) atoms by STS. Figure 6.1 shows normalized dI/dV

spectra numerically obtained from the I(V) curves taken on FeB sites. The red and

green curves indicate the representative spectra obtained on Fe(C) and Fe(H) sites

within the same scan frame, respectively. As discussed in chapter 3.2, Fe(C) atoms

are Fe3+ cations, showing a semiconducting nature. The energy-gap values of these

Fe(C) atoms are 0.4–0.8 eV. Both curves show pronounced increases of the LDOS

around at –0.4 and +0.2 eV. However, an additional occupied state is observed just

57

Chapter 6 Local electronic states of H/Fe3O4(001) surface

01.51.00.50.0−0.5−1.0−1.5

Sample Bias (V)

(dI/d

V)/

(I/V

) (a

.u.)

4

8

12

Fe(C)

−0.2 0.0

Fe(H)

OO UO

Figure 6.1 Normalized dI/dV spectra numerically obtained from theI(V) curves taken on FeB sites using the Feenstra’s method [75]. Thered and green curves show the spectra obtained on Fe(C) and Fe(H)sites, respectively. The result of Fe(C) site is equal to the one shownin Fig. 3.5. The inset shows the region around Vs = 0, in whichthe curves are offset vertically for clarity. The arrow in the insetdenotes the position of the small peak observed in the green curve.

below the Fermi level only in the green curve, as distinguished by the arrow in the

inset.

Next, the additional occupied state described above is discussed. The occupied

states are reproducibly observed on different Fe(H) sites, although the small differ-

ences in the energy position and in the density reside. In Fe3O4, features near the

Fermi level originate from 3d-3d transitions at the FeB ions with Fe2+ state [36].

Therefore, the additional occupied state can be attributed to the occupation of the

3d states for Fe2+ ions, which indicates that Fe(H) atoms exist in Fe2+-like state

58


whereas Fe(C) atoms exist in Fe3+-like state. In fact, recent DFT results of H-

adsorbed Fe3O4(001) surfaces have shown the existence of FeB 3d states just below

the Fermi level, corresponding to Fe2+-like state [37]. In addition, occupied states

just below the Fermi level provide an evidence of the reduced energy gaps near

the Fermi level of Fe(H) atoms. This result indicates that atomic-H adsorption on

a clean Fe3O4(001) surface changes the local electronic states of the surface FeB

atoms from a semiconducting to a metallic-like nature. These STS results are in

good agreement with the results of other studies of H-saturated Fe3O4(001) sur-

faces, such as the results of angle-resolved X-ray photoelectron spectroscopy [36],

which showed an increased proportion of Fe2+ ions on the surface relative to the

clean surface, and those of spin-summed metastable de-excitation spectroscopy [33]

and ultraviolet photoelectron spectroscopy [36], which showed a dramatic increase

in emission just below the Fermi level by atomic-H adsorption.

dI/dV mapping was also performed to further ascertain the occupied state just

below the Fermi level of Fe(H) atoms using the multipass scanning method shown in

Fig. 6.2 [87]. Figure 6.3(a) and (b) show a high-resolution STM and dI/dV images

of the Fe3O4(001) surface obtained using this method, respectively. A general dI/dV

VS = 1.0 V(i) VS = −0.2 V(ii)

Tip

Sample

Tip trajectory

Acquisition of topography

→ Record of tip trajectory

Detection of tunneling current

→ Acquisition of LDOS image

Sample

Tip trajectory

Figure 6.2 Schematic of multipass scanning method used in thisstudy. (i) At a sample bias voltage of +1.0 V, surface topographyis measured and recorded in the first pass using a constant-currentmode. (ii) In the second pass, at a sample bias voltage of –0.2 V,LDOS image is obtained by detecting a tunneling current using therecorded tip trajectory in the first pass. (iii) The procedure (i)–(ii)is performed in all lines of the scanning area.

59


mapping using a constant-current mode can lead to tip crashes on the surface due

to the lack of LDOS near the Fermi level [68]. In this multipass scanning method,

surface topography was measured in the first pass. During the second pass, the

acquired topographic trace was used to obtain a dI/dV image at a specific sample

bias voltage. The distance offset for the second pass was 150 pm toward the surface

with respect to the topography measured during the first pass. The STM (Fig.

6.3(a)) and dI/dV (Fig. 6.3(b)) images were obtained at a sample bias voltage of

+1.0 V and –0.2 V, respectively. Comparison of these two images clearly shows the

higher LDOS at –0.2 eV of Fe(H) than Fe(C) atoms, which is in good agreement

with the STS result shown in Fig. 6.1.

These differences in the local electronic states between Fe(C) and Fe(H) atoms can

be explained by the electron-transfer phenomenon. A differential charge density map

of the H-adsorbed Fe3O4(001) surface at the topmost (001) plane has predicted that

electrons are donated from hydrogen to surface oxygen atoms and then redistributed

toward the neighboring FeB atoms due to the saturation of oxygen dangling bonds

(a) (b)

Fe(C)Fe(H) Fe(C)

Fe(H)

Figure 6.3 (a), (b) Two-pass scan images (5.0 × 5.0 nm2). First, anunoccupied-state STM image (a) was taken at a set point of +1.0 Vand 0.3 nA. Second, a dI/dV image (b) was taken at a bias voltage of–0.2 V. The topography recorded in (a) was played with a z-offsetof 150 pm toward the surface.

60


through OH bonding [28]. Consequently, adsorbed-H and neighboring FeB atoms

should behave as electron-donor and electron-trapping sites, respectively, in analogy

with H and Ti atoms on the hydroxylated TiO2 surface [90]. Judging from DFT

results [28], two FeB atoms with an OH group neighbor are expected to gain spin-

down electrons just below the Fermi level. Since occupied states just below the

Fermi level of Fe(H) atoms are clearly resolved by this STS measurement, this

electron-transfer phenomenon is expected to occur in this Fe3O4(001) film surface,

as illustrated in Fig. 6.4. Therefore, the additional occupied state can be attributed

to the gain of spin-down electrons from the neighboring H atoms. I expect that

electron transfer from hydrogen atoms to a clean Fe3O4(001) surface can enhance

the surface electron-spin polarization. Recent theoretical works have reported that

the adsorption of carbon atoms [38] and boron atoms [39] can also be an effective

method of surface modification for achieving a half-metallic Fe3O4(001) surface. This

enhancement in the surface electron-spin polarization is also predicted to be related

to an electron-transfer phenomenon. To reveal the mechanism of these enhancements

in greater detail, further evaluation of the effect of adsorbed atoms on the surface

local electron-spin states by spin-polarized STM/STS, such as the relations between

adsorption structures and electron-spin polarization, is required.

[110]

[001]

topmost surface

N

e-

Figure 6.4 Schematic of electron transfer occurred in H/Fe3O4(001)surface. The color coding of the ions corresponds to the one shownin Fig. 5.3. The arrows indicate the directions of electron flow.

61


6.2 Correlation between OH density and surface

Fe electronic states

Figure 6.5 shows an overview STM image of the as-grown Fe3O4(001) film surface,

where FeB rows are running along the [110] or [1–10] direction on the larger terrace

[64–66]. Many bright protrusions (BPs) situated at FeB sites are observed from

place to place, as indicated in the yellow oval. As described above, the observed

BPs are Fe(H) atoms whose local electronic states are significantly modified by

surface OH groups [36, 65, 66, 74], compared to Fe(C) atoms residing in a clean

Fe3O4(001) surface. Figure 6.6(a) shows a high-resolution STM image of an area

[010]

[100]

Figure 6.5 Overview STM image (40 × 40 nm2) of the as-grownFe3O4(001) film surface. The feedback control set point was VS =+1.2 V, IT = 0.3 nA. The yellow oval encloses a representativeBP (SBP) which corresponds to paired Fe(H) atoms. The yellowarrows show the exceptional BPs (LBPs) whose length along theatomic rows is longer than SBPs. The white circles enclose theSBPs adjacently arranged perpendicular to the atomic rows.

62


containing a representative BP, in which two adjacent Fe(H) atoms are observed as

BPs. It is theoretically predicted that hydrogen atoms strongly tilt from an initial

on-top position, resulting in OH bonds between hydrogen atoms and ON atoms of

the opposite rows [37, 38]. The strong tilt of the OH bonds nearly parallel to the

surface is closely associated with the frequently observed jumping of BPs between

neighboring ON sites, which correspond to hopping of hydrogen atoms between them,

as discussed in the prior chapter [36]. Judging from the STM images, hydrogen

atoms can not be bonded to OW sites due to the nonobservation of jumping of BPs

between neighboring OW sites [65].

Another type of BPs can also be seen from Fig. 6.5, as indicated in the yellow

arrows. Figure 6.6(b) shows a high-resolution STM image of the area containing such

a BP, in which the BP is longer than that shown in Fig. 6.6(a). Henceforth, I refer to

the shorter and the longer BP as “SBP”and “LBP”, respectively. These types of BPs

~0.3 nm

N

N

W

W

(a)

Fe(H) O HFe(C)

~0.9 nm

N

W

W

NLBP

SBP

(b)

Figure 6.6 (a) High-resolution STM image (2.4 × 1.6 nm2) of anarea containing a SBP and the atomic arrangement model. (b)High-resolution STM image (3.0 × 3.0 nm2) of an area containingone LBP and two SBPs, and the atomic arrangement model. Thefeedback control set point was VS = +1.2 V, IT = 0.3 nA.

63


longer than LBPs were not observed in the STM images. The LBPs are extremely

uncommon with respect to the SBPs, resulting in only 5–10 LBPs compared to more

than 160 SBPs observed in nearly every STM measurement (wide area scan: 40 ×40 nm2 to 50 × 50 nm2) [67]. This tendency is also visible on the Fe3O4(001) surface

with a nearly saturated OH coverage (ca. 5% LBPs relative to SBPs) [74]. This

low proportion of LBPs can be attributed to the adsorption structure. The LBP

consists of four Fe(H) atoms with two OH groups neighbor, as shown in Fig. 6.6(b).

Both of two adjacent ON sites along the atomic rows are occupied by hydrogen

atoms, whereas for SBPs either ON site is occupied [67]. As a result, LBPs are

energetically less favorable than SBPs due to the stronger OH–OH electrostatic

repulsion at the shorter distance between the nearest OH groups, corresponding to

much less emergence in the STM images. DFT results also indicate the enhanced

instability of the surface with increasing OH coverages due to the OH–OH repulsion,

therefore maximized OH–OH distance that reduces it is preferable [37]. Here, note

that the SBPs adjacently arranged perpendicular to the atomic rows are observed

in the STM images, as enclosed by the white circles in Fig. 6.5 or the white ovals in

Fig. 6.6(b). This adsorption configuration can also be energetically unfavorable due

to the OH–OH repulsion, resulting in infrequent cases observed in the STM images

(not more than 10 cases observed in the 40 × 40 nm2 STM images) [67].

Figure 6.6(b) also shows the central Fe(H) atoms within the LBP have a slightly

increased apparent height compared to those within the SBPs (ca. 15 pm), which

reflects the differences in the local electronic states between them [67]. I now turn

to discuss the differences in the local electronic states between Fe(H) atoms within

SBPs (Fe(H1)) and central Fe(H) atoms within LBPs (Fe(H2)). To investigate this

difference, I probed the LDOS of these two types of Fe(H) atoms by STS. Figure

6.7(a) shows a high-resolution STM image of an area containing both Fe(H) atoms

in which the STS measurements were performed. The green and blue curves in

Fig. 6.7(b) show the normalized dI/dV spectra obtained on Fe(H1) and Fe(H2) site

indicated in Fig. 6.7(a), respectively. Figure 6.7(b) also shows the normalized dI/dV

spectrum taken on a Fe(C) site situated far from OH groups [66]. The normalized

64


(a)

N

W

N

N

W

W

LBP

SBP

(b)12

10

8

6

4

2

0

(dI/d

V)/

(I/V

) (a

.u.)

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

Sample Bias (V)

-0.2 0.0

Fe(H1)

Fe(H2)

Fe(C)

Fe(H1)

Fe(H2)[010]

[100]

Figure 6.7 (a) High-resolution STM image (3.0 × 3.0 nm2) of an areacontaining Fe(H1) and Fe(H2) atoms, and the atomic arrangementmodel. The feedback control set point was VS = +1.2 V, IT = 0.3 nA.(b) Normalized dI/dV spectra obtained on two FeB sites indicatedin panel (a) and on a Fe(C) site [66]. The normalized dI/dV spectrawere numerically obtained from the I(V) curves using the Feenstra’smethod [75]. The inset shows the region around Vs = 0.

dI/dV spectra were numerically obtained from the I(V) curves using the method

proposed by Feenstra [75]. The three I(V) curves intersect the imaging set point at

{+1.2 V, 0.3 nA} (data not shown), i.e., it is certain that there is no variation of

the tip-sample distance during the STS measurements. Pronounced peaks around

at Vs = –1.0 V are clearly observed in all three spectra, as shown in Fig. 6.7(b).

These peaks can be attributed to the occupation of 3d states for Fe3+ ions [66].

However, the two spectra obtained on Fe(H1) and Fe(H2) sites also show additional

small peaks just below Vs = 0, whereas this peak is not observed in the spectrum

of the Fe(C) site (see the inset). As discussed in chapter 6.1, the observed peaks

are attributed to the occupation of 3d states for Fe2+ ions, and this difference in

the local electronic states between Fe(C) and Fe(H) atoms can be explained by the

electron-transfer phenomenon described above [66]. This phenomenon is expected

to occur in this Fe3O4(001) surface, giving rise to the observed peaks just below Vs

65


= 0 of two spectra obtained on Fe(H1) and Fe(H2) sites.

Figure 6.7(b) also shows an enhanced intensity of the peak of the Fe(H2) site

with respect to that of the Fe(H1) site, which means the higher LDOS just be-

low the EF of Fe(H2) atoms relative to Fe(H1) [67]. As described above, given

that the modifications of FeB electronic states are induced mainly by surface OH

groups, this STS result demonstrates that Fe(H2) atoms receive more contributions

from OH groups than Fe(H1). Since Fe(H2) atoms are surrounded by two neigh-

boring OH groups within the atomic rows relative to Fe(H1) with an OH group

neighbor, as depicted in the atomic arrangement model in Fig. 6.7(a), one should

consider Fe(H2) atoms gain more electrons from hydrogen atoms than Fe(H1) [67].

The gain of more electrons can be speculated to lead to the enhanced LDOS just

below the EF. This enhanced LDOS with increasing neighboring OH groups can

also be closely associated with the previous experimental reports, which show the

enhanced electron-spin polarization of Fe3O4(001) surfaces with increasing H cov-

erages [33–35]. Since electronic states near the EF of FeB atoms are dominated by

spin-down electronic states [30,69,80], the enhanced LDOS just below the EF would

be due to the gain of more spin-down electrons from hydrogen atoms. Therefore, the

experimental observations of the enhanced electron-spin polarization near the EF of

Fe3O4(001) surfaces with increasing hydrogen coverages [33–35] should be derived

from the increase of half-metallic-like FeB atoms, which is induced by the increase of

ON sites occupied by hydrogen atoms. These results indicate that the saturation of

ON sites by hydrogen atoms can change all surface FeB atoms from a semiconducting

to a half-metallic nature. In addition, these experimental findings have significant

implications for understanding the correlation between hydrogen coverages and FeB

electronic states. These STS results obtained on Fe(H1), Fe(H2) and Fe(C) sites

have revealed that surface OH density is a factor for determining the LDOS just

below the Fermi level of surface FeB atoms [67].

66


6.3 Summary

STM/STS measurements and dI/dV mapping of H/Fe3O4(001) surfaces were per-

formed to investigate the surface local electronic states. STS results obtained on two

types of FeB sites, one with neighboring OH groups and the other without, indicated

that adsorbed-H atoms can modify the LDOS around the Fermi level of neighbor-

ing FeB atoms. The energy-gap value of these FeB atoms is reduced because of the

occupation of 3d states just below the Fermi level, corresponding to Fe2+-like state.

These modifications of local electronic states can be due to the gain of electrons from

neighboring hydrogen atoms. These results indicate that adsorbate-induced mod-

ifications in the electronic and electron-spin properties of the topmost surface will

play a significant role in designing interfaces for spintronic devices, which require

high electron-spin polarization.

In addition, correlation between surface OH density and surface FeB electronic

states were also investigated by STM/STS. Two types of BPs with different lengths

along the atomic rows are observed in the STM images. The shorter BP consists

of two FeB atoms with one OH group neighbor, on the other hand, the longer

BP consists of four FeB atoms with two OH groups neighbor. The latter adsorption

configuration where the OH–OH distance is shorter gives rise to much less emergence

in STM images, presumably due to the stronger OH–OH electrostatic repulsion. In

addition, the STS results evince the higher LDOS just below the Fermi level of

surface FeB atoms with increasing neighboring OH groups. These experimental

findings indicate that the LDOS just below the Fermi level of surface FeB atoms can

be tuned by varying surface OH densities.

67


68

Chapter 7

Summary

This research work produced following six important results and knowledges.

1. Scanning tunneling microscopy (STM) studies of a clean Fe3O4(001) surface

showed the B-terminated surface with a (√2×

√2)R45◦ reconstruction, which is

in agreement with the previous works. Scanning tunneling spectroscopy (STS)

studies of the surface revealed its energy gaps of 0.4–0.8 eV, indicating a semi-

conducting nature relative to the predicted half-metallic one of bulk Fe3O4.

Moreover, it was found that the surface local electronic states of step edges

show a metallic-like nature with respect to a semiconducting one of terraces,

which can be due to the presence of more oxygen vacancies and more adsorbates.

2. Fe3O4(001) subsurface electronic structures were investigated by STM/STS and

differential tunneling conductance (dI/dV) mapping. The STS results showed

the higher local density of states (LDOS) just below the Fermi level of nar-

row sections than wide ones of the surface reconstruction perpendicular to the

FeB rows. The dI/dV map clearly showed charge-ordered electronic structures

and reproduced the STS results. These observed structures were consistent

with a subsurface charge-ordering model of Fe2+-Fe2+ and Fe3+-Fe3+ dimers,

as proposed in previous density functional theory studies. These experimental

findings reveal the origin of the (√2×

√2)R45◦ surface reconstruction to be the

subsurface charge-ordered electronic structures.

3. It was demonstrated by X-ray photoelectron spectroscopy and STM that the

bright protrusions (BPs) observed in the STM images of ultra-high vacuum

69

Chapter 7 Summary

(UHV)-prepared Fe3O4(001) film surfaces can be due to surface OH groups. In

addition, the number of OH groups was found to increase with increasing UHV

holding time of Fe3O4(001) film samples, which indicates that the OH groups

are derived from UHV residual gases.

4. STM studies of H/Fe3O4(001) surfaces have revealed that surface FeB atoms

relax toward the bulk-terminated positions by OH groups formed between hy-

drogen and surface ON atoms. Moreover, adsorption-site dependences for hy-

drogen atoms were investigated by STM measurements in detail, and two types

of surface ON sites were found to be almost equivalent for hydrogen adsorption,

which is different from the previous theoretical prediction.

5. Effect of hydrogen atoms on the surface FeB electronic states were investi-

gated by STS and dI/dV mapping. The STS spectra measured on FeB sites

with and without neighboring OH groups, and dI/dV map have indicated that

adsorbed hydrogen and their neighboring FeB atoms behave as electron-donor

and electron-trapping sites, respectively. This electron-transfer phenomenon

changes the FeB atoms from a semiconducting to a metallic-like nature. These

local electronic state modifications are related to the occupation of Fe 3d states

just below the Fermi level, corresponding to Fe2+-like state. The orbital occu-

pation can be explained by the gain of electrons from the hydrogen atoms.

6. Correlation between surface OH density and FeB electronic states were inves-

tigated by STM/STS. Two types of BPs, whose lengths along the atomic rows

are different, are observed in the STM images. The shorter and longer BPs con-

sist of FeB atoms with one and with two OH groups neighbor, respectively. In

addition, STS studies show the higher LDOS just below the Fermi level of FeB

atoms with increasing neighboring OH groups. The variation can be derived

from the difference in the gain of electrons from hydrogen atoms, which is due

to the difference in the number of neighboring OH groups. These STM/STS

results reveal that surface OH density is a factor for determining the LDOS just

below the Fermi level of surface FeB atoms.

70

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82

Publication List

　

(1) A. Ikeuchi, S. Hiura, T. Mizuno, E. Kaji, A. Subagyo, and K. Sueoka: “Atom-

ically Resolved Observations of Antiphase Domain Boundaries in Epitaxial

Fe3O4 Films on MgO(001) by Scanning Tunneling Microscopy”, Jpn. J. Appl.

Phys. 51, 08KB02 (2012).

(2) T. Mizuno, H. Hosoi, A. Subagyo, S. Oishi, A. Ikeuchi, S. Hiura, and K. Sueoka:

“Noncontact Atomic Force Microscopy Observation of Fe3O4(001) Surface”,

Jpn. J. Appl. Phys. 51, 08KB03 (2012).

(3) S. Hiura, A. Ikeuchi, S. Shirini, A. Subagyo, and K. Sueoka: “Scanning Tunnel-

ing Microscopy Study of an Altered Fe3O4(001) Thin Films Surface by Hydrogen

Adsorption”, e-J. Surf. Sci. Nanotech. 12, 26 (2014).

(4) S. Hiura, A. Ikeuchi, S. Shirini, A. Subagyo, and K. Sueoka: “Effect of adsorbed

H atoms on the Fe electronic states of Fe3O4(001) film surfaces”, Phys. Rev. B

91, 205411 (2015).

(5) S. Hiura, A. Ikeuchi, S. Shirini, A. Subagyo, and K. Sueoka: “Correlation be-

tween OH density and Fe electronic states of H/Fe3O4(001) film surfaces stud-

ied by scanning tunneling microscopy/spectroscopy”, Jpn. J. Appl. Phys. 54,

08LB02 (2015).

(6) S. Hiura, A. Ikeuchi, M. Jochi, R. Yamazaki, S. Takahashi, A. Subagyo, and

K. Sueoka: “Direct observation of subsurface charge ordering in Fe3O4(001) by

scanning tunneling microscopy/spectroscopy”, to be submitted.

(6) S. Hiura, M. Jochi, R. Yamazaki, S. Takahashi, A. Subagyo, and K. Sueoka:

83

Publication List 　

“Direct visualization of the occupied states near the Fermi level in H/Fe3O4(001)

surface using scanning tunneling microscopy”, in preparation.

84

International Confererence

　

(1) Y. Okabe, S. Hiura, A. Ikeuchi, A. Subagyo, and K. Sueoka: “Origin of negative

differential conductance observed on Cs adsorbed InAs(110) surfaces”, Interna-

tional Scanning Probe Microscopy Conference (ISPM 2011), p.86, P14, Munich,

Germany, June, 2011.

(2) S. Hiura, A. Ikeuchi, A. Subagyo, and K. Sueoka: “Difference in electronic states

between clean and modified Fe3O4(001) thin film surfaces”, The 20th Inter-

national Colloquium on Scanning Probe Microscopy (ICSPM20), S3-53, p.91,

Naha, Japan, December, 2012.

(3) S. Hiura, A. Ikeuchi, A. Subagyo, and K. Sueoka: “Change of electronic states

induced by hydrogen adsorption in Fe3O4(001) thin films surface”, The 12th

Asia Pacific Physics Conference (APPC12), A1-PWe-10, p.223, Chiba, Japan,

July, 2013.

(4) S. Hiura, A. Ikeuchi, A. Subagyo, and K. Sueoka: “STM/STS studies of mod-

ified magnetite thin films surface on MgO(001) substrate”, International Con-

ference on Nanoscience and Technology (ICN+T 2013), SS-P1-17, p.83, Paris,

France, September, 2013.

(5) S. Hiura, A. Ikeuchi, S. Shirini, A. Subagyo, and K. Sueoka: “Structural and

Electronic Properties of a Modified Fe3O4(001) Films Surface”, 12th Interna-

tional Conference on Atomically Controlled Surfaces, Interfaces and Nanos-

tructures (ACSIN-12) & 21th International colloquium on Scanning Probe Mi-

croscopy (ICSPM21), 7PN-111, p.190, Tsukuba, Japan, November, 2013.

(6) S. Hiura, A. Ikeuchi, A. Subagyo, and K. Sueoka: “Atomic structures and local

electronic properties of modified Fe3O4(001) film surfaces”, The 59th Annual

85

International Confererence 　

Magnetism and Magnetic Materials (MMM) Conference, GU-07, p.261, Hawaii,

USA, November, 2014.

(7) S. Hiura, A. Ikeuchi, A. Subagyo, and K. Sueoka: “Effect of adsorbed-H atoms

on Fe atoms in Fe3O4(001) film surfaces”, 22nd International Colloquium on

Scanning Probe Microscopy (ICSPM22), S4-7, p.45, Atagawa, Japan, Decem-

ber, 2014.

(8) S. Hiura, M. Jochi, A. Subagyo, and K. Sueoka: “Scanning tunneling microscopy

/spectroscopy study of magnetite film surface: Effect of hydrogen atoms on the

surface structural and electronic properties”, JSPS workshop on Japan-Sweden

frontiers in spin and photon functionalities of semiconductor nanostructures,

Jozankei, Japan, August, 2016.

(9) S. Hiura, R. Yamazaki, M. Jochi, S. Takahashi, A. Subagyo, and K. Sueoka:

“Scanning Tunneling Spectroscopy Study of H/Fe3O4(001) Surface”, 24th In-

ternational Colloquium on Scanning Probe Microscopy (ICSPM24), accepted

for presentation.

86

Domestic Confererence

　

(1)樋浦諭志, 岡部泰志, 池内昭朗, Agus Subagyo, 末岡和久: “Cs/InAs(110)表面に

見られる微分コンダクタンスの探針依存性”, 第 72回応用物理学会秋季学術講

演会, 30a-ZC-5, 山形, 2011年 8月.

(2)樋浦諭志,池内昭朗, Agus Subagyo, 末岡和久: “Fe3O4(001)薄膜表面に見られる

吸着構造の STM/STS測定”, 第 73回応用物理学会秋季学術講演会, 11a-PA3-1,

松山, 2012年 9月.

(3) S. Hiura, A. Ikeuchi, A. Subagyo, and K. Sueoka: “STM/STS measurements of

a modified epitaxial grown Fe3O4(001) films surface”, 第 11回スピントロニク

ス入門セミナー・若手研究会, P11, 札幌, 2012年 12月.

(4)樋浦諭志, 池内昭朗, シリニ・ソライヤ, スバギョ・アグス, 末岡和久: “表面修飾

された Fe3O4(001)薄膜表面の STM/STS測定”, 第 37回日本磁気学会学術講演

会, 5pD-1, 札幌, 2013年 9月.

(5)樋浦諭志, 池内昭朗, Shirini Soraya, Subagyo Agus, 末岡和久: “吸着水素により

誘起された Fe3O4(001)表面の局所電子状態測定”, 平成 25年度日本表面科学会

東北・北海道支部学術講演会, P-06, 仙台, 2014年 3月.

(6)樋浦諭志, 池内昭朗, Shirini Soraya, Subagyo Agus: “Fe3O4(001)薄膜表面にお

ける吸着水素により誘起された鉄イオン還元”, 第 61回応用物理学会春季学術

講演会, 18p-PG9-4, 相模原, 2014年 3月.

(7)樋浦諭志,池内昭朗, Subagyo Agus,末岡和久: “水素吸着によるFe3O4(001)表面

の局所電子状態変化の研究”, 第 75回応用物理学会秋季学術講演会, 18p-PA8-2,

札幌, 2014年 9月.

(8)樋浦諭志,池内昭朗, Shirini Soraya,城地雅史, Subagyo Agus,末岡和久: “STM/STS

による水素吸着 Fe3O4(001)薄膜表面の局所電子状態測定”, 第 76回応用物理学

87

Domestic Confererence 　

会秋季学術講演会, 16a-2U-5, 名古屋, 2015年 9月.

(9)樋浦諭志,山崎陸,城地雅史,高橋聡太朗, Subagyo Agus,末岡和久: “H/Fe3O4(001)

表面におけるフェルミ準位直下の局所状態密度測定”, 第 63回応用物理学会春

季学術講演会, 21p-H113-1, 東京, 2016年 3月.

88

study on atomic and local electronic structures of fe3o4 ... · chapter 4 electronic structures of...

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